I personally don't see this happening in San Francisco, at least not with top-tier companies. The only workers I've seen leaving SF have moved to NYC for adventure, or moved back to China.
Also as a single person housing is flexible for most. If you rent a room in a house in SF (most apartments are houses) you’d most likely be spending less than if you rented a 1 bedroom in LA.
And the houses are (slightly) cheaper, even in Burbank, than in the SF Bay Area.
I have a 2 bedroom place in a very nice part of Los Angeles for less than my brother pays for a 1 bedroom studio apartment in a okay part of Silicon Valley. My commute is the same distance as his, but mine only takes an hour (rail + walking) while his takes 1.5 hours or more each way (driving). I don't need a car in LA to get to most of the major destinations; he can't travel around the Bay Area without one.
The city works if you are able-bodied without kids. In that case, you walk as far as 11 miles but usually just a few miles. It also sort of works for the people with $2,000,000 single-family homes, because driving to a distant supermarket (with parking) becomes an option.
Totally agreed that there's no place to park btw, yet another difficult thing about that city.
When I lived in the Sunset, I took the train, and that was worse than the bus - overcrowded trains meant that they sometimes skipped stops. So I usually biked instead.
The city is just not that big, most places are within walking distance if you're not going to/from far-flung areas of the city -- you can walk from the Marina to the Financial District in under an hour. (but the 30X made it a tolerable commute, usually under 30 minutes)
I'm just comparing the walkability to other major cities. I find SF to be pretty unwalkable all things considered. Especially compared to European cities or NY.
LA has a very good bus network within the city proper--one of the biggest in the world. It's possible to get within 1 mile of any publicly-accessible address by bus, and to neighboring cities (i.e., Santa Monica, Long Beach) and enveloped cities (i.e., Beverly Hills, Inglewood) as well.
LA's rail also enables car-free travel between Santa Monica, Culver City, Long Beach, North Hollywood, Hollywood, Pasadena, and Downtown. Including regional rail (i.e., the BART equivalent), it's possible to get as far out as Ventura County to the north, San Bernardino and Riverside to the east, and Disneyland/Orange County and San Diego County to the south.
Really depends where in the bay you are -- there are definitely areas outside of San Francisco you can do without a car. I lived in Berkeley and later Oakland without a car (probably four years total), and honestly it was pretty damn nice not having to deal with one. Menlo Park on the other hand? You'll probably need a car.
It's also surprisingly bikeable. Could there be more bike lanes? Of course. More are always coming at least. But when an artery is backed up (which it usually is), you can slither along the side, and with the grids there are usually residential routes that you can just fly along.
Unless you are one of the lucky few that has struck it really rich, a lot of people who turn late 20s/early 30s, get married, and have or want kids cannot afford to stay in the Bay Area even if they make a well above average income.
I'd count true high-tech among them personally (Waymo, for example), but would not count adtech (Google, Facebook) or employers known to be unpleasant to work for (Amazon, Netflix).
Also, it may be that the workers moving away from places like SF are just not people who define themselves by the name brand of their employer and just want better overall quality of life.