What happens when the Silicon Valley boom is over? What happen to those neighborhoods? I guess that's not your problem.
Generally areas regarded as having a vibrant culture also correlate with high percentages of renters -- because they're in cities. Areas with high percentages of homeowners and low percentages of renters tend to be suburbs.
In any event we can answer your question of what happens when the boom is over because we've already lived through two tech booms in this area. The answer is: Prices fall and folks with investment properties will lose some money but not much else changes. Homeowners get hurt a lot more than renters when bubbles pop.
When the supply of SF housing overtook demand in the late 1960s, and this oversupply led to incredibly-affordable rooms, it brought about the Summer of Love.