It will take some time, and it may be decades or years before anything changes, but evenually ISIS will be defeated in Syria. Whether this is because the ISIS movement dies down and people stop joining or followers leave or it is becasue an outside force defeats them and wipes them out, this will not last forever.
ISIS primarily uses fear as their tactic. People will live in fear for only a limited time before they take decisive action to eliminate the threat. Increased international attention of what is taking place in Syria will ultimately lead to ISIS being defeated there and in many other countries where it currently has a stronghold.
ISIS will not likely be defeated in Syria. Yes, the terrorists organization could be pushed back and cut down to size. However, it is unlikely that many countries will step up to commit the resources needed to defeat the terrorist group. Instead, ISIS will probably be somewhat contained, but never fully defeated.
I think that it all depends on the situation and the circumstances at hand. I don't think that anyone can be able to say whether or not they will be defeated. It really is a matter of what happens in Syria and the conditions at the time. Syria may not have things together as expected.