People are not "fleeing" other cities to come to SF because it's undense. They come to SF because SF has jobs and it's nice to live here. I'm somewhat unconvinced that more density in SF would make it less nice for all the people coming here.

High prices is not beneficial to the wealthy, it's only beneficial to homeowners. A vast majority of tech workers in SF are renters, who can afford high rents but would nonetheless prefer them cheaper.