Because if "tech elites" actually are leaving, then the increase in demand will have been proven to be transient

Surely many developers would have made the bet anyway, at least in 2010-2019, but going forward it'll be worth considering whether building housing for people who are likely to leave en masse is worth the risk.

"transient" for 30 years? It takes much less to gather investments, build houses and sell them.

People are building houses in countries where they sell at 20x lower prices.

Unless construction workers in SV are paid 1 million $ / year, the reason for not building is not that it's not (hugely) profitable.

Rents are still among the highest in the country. There is plenty of demand.