This makes sense, SF benefited a lot from the social/mobile/on demand boom. Silicon Valley is set to benefit more from the AI/ML hype as this as stronger ties to academia, and also older, more academic type engineers who don't get as much value from living in the denser city of SF (that the mobile/social/on demand startups did).

Long term both cities will dominate and I don't see this as a structural change, this just seems like a shift based off of what is popular in VC funding right now.