I think the millennials are settling into better jobs like Google and Facebook and the younger people are flocking to startups. That might be the reason why startups are now focusing on SF rather than SJ area.
The Millenial generation really has three sub-generations: Xennials (1981-1986, distinguished because they can a. remember the Cold War and b. graduated into a decent economy, so they don't have the economic despair of the rest of Millenial generation), older Millenials (1987-1992, these are the folks who were totally fucked over by the Great Recession), and younger Millenials (1991-2000, young enough that they could actually adapt to the post-2009 economic conditions and make life-choices accordingly). Almost all Millenial billionaires are actually Xennials: Mark Zuckerburg, Brian Chesky, Drew Houston, Kevin Systrom, Mary Kate & Ashley Olsen, et al. The older Millenials are who people usually think of as "Millenials" (grew up expecting a lot, graduated when not a lot was to be found). There's a marked difference in behavior with young Millenials: they are much more studious, much more career-focused, more of them went into STEM careers, they're the ones driving the FIRE & cryptocurrency movements, etc.
Also, this group of xennials might not have graduated in a good economy (i.e. around 2001)