I'm not sure this theme will survive much longer. It seems no coincidence that the founders of the two most successful YC companies, Dropbox and Airbnb, weren't interested in any "big exit". (Drew remarked on his YC app that he'd find it difficult to turn down a million dollars for 6 months of work, but even that reads like he probably would've passed on such an offer.)
The most promising founders seem not to be interested in exiting.
Greed for a huge exit. Fear of being overtaken by a VC funded competitor.
I think it's a legitimate concern.