I don't really see it affecting say Atherton or Los Altos Hills much - the people who live there are rich enough that splitting a lot and putting up duplexes isn't going to make a measurable economic difference to them, at least not one worth giving up their prime estate. The hillside communities (mid-peninsula, Oakland/Berkeley hills, Saratoga/Portola/Woodside) likely also won't see much adoption, just because the geology & topography doesn't really support the types of construction & road access to build multi-family housing.

I could see it making a huge difference in "affluent suburbia" - places like Sunnyvale, Cupertino, San Jose, the SFH parts of Mountain View and San Francisco, etc. The lots there are generally wide enough to support a lot split, and you have a lot of homeowners that just spent $3M because you need to in order to own property, carrying a massive mortgage. The lot split would immediately let them pay back a good chunk of that principal, and a duplex would give you rental income to cover most of the payment. A lot of homeowners would go from being financially stretched on 2 FAANG incomes to being comfortably able to live on one salary, while still maintaining ownership of a duplex.

Many of these neighborhoods are precisely the ones that need it most - they're close to jobs, they're flat, they could support a lot more population, and they already have significant pro-housing contingents. Traffic on 85 is going to suck even more than it already does, though.