Just like the two friends being chased by the bear -- you don't need to be faster than the bear, you just need to be faster than your friend.

perfect competition, symmetric monopolistic competition, for example. it is done all the time in this literature.

I understand that. But these are simplifying assumptions that make the modeling easier. The question is are these assumptions good enough to represent the real world? My prior on this is no, but I'm open to being shown that I'm wrong.