To the extent that there is a military solution to the Syrian problem, it would revolve around the use of the Turkish Army. The combination of American air power and Turkish infantry would be more than a match for Assad’s forces and IS, and the ultimate political resolution could be left to the Turks, who presumably would be happy to rearrange the affairs of their neighbors to their liking.
There are basically two reasons why this hasn’t happened. First, the Turks do not appear to believe that the benefits from reshaping the Syrian government and eliminating IS are worth the risks (a confrontation with the Russians being one of them). Second, the Turkish government, for domestic political reasons, puts a much higher priority on fighting Kurdish militants than on getting rid of Assad and IS. That puts them in conflict with US policy, which generally supports the aspirations of the Kurds, who are viewed as our most reliable ally.
The only way this equation is going to change is if IS starts becoming much more destructive within Turkey, so the government has to change its priorities. That isn’t impossible, based on recent events, but it isn’t likely, either.