yep, once all sentries are down, not much point of being spy anymore.
Not true at all. Spy can be
very useful for taking out high value targets and going for point caps. The key is that you only need one,
maybe two spies if they're both really good but anymore than that and it becomes an overall disadvantage because you have less guys fighting on the front lines.
After thousands of hours of spy play under my belt, I don't even consider the class a counter to sentries. Sure it
can do that, but you have soldiers and demomen who are more suited to that role. Spy is the class to take out that enemy medic building uber, or to pick off lone players getting back to the front lines. Another great use is to work with a heavy medic pair by staying cloaked ahead of them waiting for enemies to engage your teammates so you can backstab them.