Syrian Rebels who used to fight ISIS are now joining ISIS to defeat Assad

Another dimension is that success begets success. Increasing numbers of Syrian rebels, including those who were fighting ISIS until recently, are now shifting sides, because they think the wind is blowing in ISIS's way. This is slowly affecting the orientation of ISIS. With more Syrian rebels joining, ISIS is likely to shift its priorities and to devote more time and resources to the fight against Bashar al-Assad – even though this was not its original priority.

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How Entrenched Is ISIS in Syria?

This also might be one reason Europe and the U.S. aren't rushing into Syria right now to fight ISIS.

However, another way to look at this would be that if Assad is defeated, Syria might not have a functional government ever for 20 to 30 years. So, another way to see this is that Assad might be "The only show in town" in regard to having any government at all in Syria during the next 20 to 30 years. 

Syria, with Assad gone is going to look exactly like Libya does now for years to come.

However, if Assad is defeated this helps the U.S. and Europe in regard to shaming  Assad, Iran and Russia.

So, any way you look at this things in Syria likely will get much worse for the Syrian people soon. 

Sadly, this is what a Cold War looks like. It was like this in many parts of the world with millions of people dying various places from 1945 until 1991.