I don't think that's what those people meant. YC-style investments were seen by many as a way to attract talent to an area. I viewed such predictions as saying that regional YC's would fail to attract talent. I don't see why anyone would think that people who are already in the city would all leave just because they've participated in such a program. If anything, it would make them less likely. Having to leave an investor is a disincentive to moving.