Why is a "Homeless March on Washington" needed to force the implementation of a Basic Income Guarantee? The question is not whether Basic Income Guarantee is either possible or desirable. The question is why any kind of force should become necessary to implement such a simple and reasonable idea. The second installment of this series closed with such questions, and this final installment will outline some possible answers to the riddle. 
The most general answer is that our current economic system – Capitalism – is by far the most formidable barrier to implementing a Basic Income Guarantee, hereafter referred to as "BIG". Richard C. Cook and many others throughout American history have effectively maintained that BIG is entirely possible, rendering homelessness and unemployment both unnecessary and inexcusable. Many others, especially those who now find themselves unemployed or homeless or both, certainly consider BIG desirable. Meanwhile, Dr. Martin Luther King Jr. suggests, "deep structural change" will be necessary to implement a Basic Income Guarantee and to eradicate poverty overall.  This makes sense intuitively, but it also raises at least one other important question: Should "deep structural change" precede the implementation of BIG or follow it?
All this in mind, let's briefly examine how our economic system works and explore how and why BIG might change that system to discover why a "Homeless March on Washington", among other extreme measures, might become necessary to force its implementation. To begin, Richard C. Cook maintains, "People simply do not earn anywhere near enough to buy what the economy produces".  Others argue that "those who produce the goods and services of society are paid less than their productive contribution". 
So what's the difference? Do we overproduce, or are we underpaid? Is the glass half full or is the glass half empty? Does it really matter? Technically, it does. But in a general sense, it doesn't -- because either way the result is nearly always the same: Unearned income is siphoned from producers daily and hoarded by wealthy non-producers, eventually resulting in some form of economic crisis characterized by rising levels of unemployment.
A Basic Income Guarantee would convert the life-threatening conditions of "unemployment" to more livable and less fearsome conditions of "leisure". Instead of working two or three minimum-wage jobs just to pay the rent and keep the family fed, Americans might work one part-time (or full-time) job, and still have plenty of time to spend with their families and for self-improvement like continued education or vocational training. Instead of living in fear of job displacement, every American worker would have a permanent safety net to fall back on in the case of job loss, illness, injury, maternity, etc. As a "birthright heritage" all American citizens, including children, would be guaranteed a monthly income to provide for their basic needs -- regardless of employment status.
Moreover, Richard C. Cook suggests that technological advancement makes BIG an economic imperative:
"In an advanced mechanized economy, fewer workers are needed to produce the same amount of goods. This should result in a societal “leisure dividend” but instead puts people out of work and forces them to compete for the remaining service economy jobs. There are estimates that by 2030 robots will take over fifty percent of the jobs in the U.S. economy." 
But others suggest unemployment is a structural feature of Capitalism.  Like the supporting beams of a building, unemployment is not an accident that can or should be corrected or removed under the terms and conditions of the current economic system. As long as labor is a cost of production, employers are highly motivated to drive wages as close to zero as possible in order to maximize profit. Therefore, conditions of unemployment must remain as unpleasant and unattractive as possible to discipline the existing workforce and drive wages down. With these unspoken rules in place, workers are typically forced to accept nearly any sort of employment for any available wage in order to avoid the perilous conditions of unemployment. If unemployment or the fear of unemployment is removed, the system will collapse.
So in a very real sense, there is a lot of incentive under Capitalism for employers and political leaders to restrict or actually prevent people from getting their basic needs met. This is the main reason why the system itself -- Capitalism -- is the most formidable barrier to the implementation of Basic Income Guarantee. If unemployment is converted to leisure as Richard C. Cook suggests, then workers are no longer forced to sell their labor to employers in exchange for "wages" just to get their basic needs met.
Does this mean BIG is impossible? No, not at all. But once again, “deep structural changes” will probably be necessary for its implementation. In fact, author and professor of philosophy, Michael Howard, outlines some pragmatic advantages that might result from implementing a Basic Income Guarantee:
“Work sharing would become more feasible, because a full-time job would not be necessary to make ends meet; this would tend to reduce unemployment. The coercive nature of the employment contract would be undermined: no one would be forced to work for wages out of economic necessity. A possible further consequence is a rise in income for unattractive, dangerous, or dirty work, because the economically desperate who now take such jobs at minimum wage would be free to refuse such work. Work conditions generally might be made more attractive, and work itself more intrinsically rewarding, as a way of attracting workers. A floor of income for everyone would make possible a deregulation of the labor market and greater flexibility and innovativeness in the economy.” 
Such results might seem very attractive from a labor perspective, but dubious or even threatening from an employer point-of-view. If workers are no longer forced to sell their labor to employers in exchange for basic survival and they are no longer willing to accept any wage that happens to be available, then employers risk losing absolute control of the workforce and wages and therefore profit.
So if structural change precedes the implementation of BIG, it will most likely be tinkering around the edges organized from the top down in the interest of employers and others who stand to lose the most from implementing BIG. As such BIG might never actually be implemented at all. We already see this in Barack Obama's promises to "create more jobs" to save the failing "American middle class". But for a number of reasons, Obama's promises fall short short of the "American Dream", even if he successfully implements them.
First, there is no clear definition for "American middle class".  So it's very easy for Obama to issue rhetorical claims in this regard when so many Americans with annual incomes ranging from $30k to $250k tend to consider themselves "middle class". Second, Ralph Nader insists the "American middle class" is not the most urgent problem when easily 40-percent of American households live in "poverty".  Many nations seem to define "poverty" more clearly than "middle class", as Wikipedia estimates half the world population currently lives in poverty under the dominance of Capitalism.  Third, Barack Obama observes that the new jobs being created tend to pay far less than the old jobs that have been lost by American workers.  This corporate "race to the bottom" -- driving American wages as close to zero as possible -- is no more "accidental" than either unemployment or the current economic crisis overall. Moreover, Obama's plan to "create more jobs" doesn't "save the American middle class", it merely forces more Americans of every skin-color and race further into poverty.
Conversely, if BIG is implemented immediately, then the needs of every citizen will be satisfied right away, forcing employers and other "leaders" to readjust accordingly. Some enterprises might be forced to raise wages. Others might go out of business. Still others might find they can actually reduce wages in proportion to the dividend provided by BIG. The smartest firms will reorganize more cooperatively -- removing wages and employers altogether -- to thrive within a new socioeconomic structure. In this view, Basic Income Guarantee could be an effective strategy to initialize or even "revolutionize" a needed transition from the instabilities and inherent contradictions of Capitalism toward more balanced and sustainable conditions of genuine Economic Democracy.
But all of the preceding information posits a fundamental choice: If American workers honestly prefer corporate domination, then American workers must also adjust their lifestyles to accept the lower wages that are currently being paid in countries like Mexico, China, India and the Philippines. If this seems unacceptable to American workers, then at least one alternative is to reorganize American workplaces more regionally, democratically and cooperatively to eventually reject the dominance of global corporations altogether.