To begin with, the primary demand that Assad has to go before there can be serious talks is aimed at torpedoing any prospect of negotiations. No one -- least of all Assad -- is going to negotiate his own demise, and the Syrian Army and the country's Alawite, Christian and Druze minorities know exactly what will happen to them if the Damascus regime collapses. The Nusra Front may not as brutal as the IS, but al-Qaeda only looks good if your standard of comparison is the Islamic Front. Anyone who believes the "moderates" will take over should consider unicorn hunting as a profession.
In the long run Assad should go, and one suspects that Syrians will vote him out at some point. But the "out first" demand is just a way to continue the war. The only real hope is a ceasefire and a national unity government representative of Syria's enormously diverse population. An arms embargo on all parties, and a commitment to block fighters infiltrating the country would encourage the parties to step back from the current stalemate and consider negotiations.
Will that get rid of IS? Nope. The Islamic State is an actual state, with a large population, a lot of whom are not just waiting to rise against their Islamic captors. The IS is brutal -- though the Arabs suffered far more deaths in the invasion of Iraq -- but it is not corrupt. To imagine that the inept and corruption-riddled Iraqi Army is up for a serious scrap is delusional.
The Shiite militias are tough and capable, but also very sectarian, and many Sunnis simply don't trust them.
The Turkish Army does not want to go into Syria, and there is zero support in any Western country for a replay of Iraq and Afghanistan. On top of which, a U.S. or NATO invasion is exactly what the IS would like to provoke. Ironically, the only force that could possibility defeat the Islamic State is the Syrian Army. Getting from here to there, however, will require a diplomatic sea change in the region. But one thing is certain: the current U.S.-Turkish "plan" will make everything worse.
How do these crackpots come up with this stuff?