Outside of work reasons I don't know many people who actually travel between these cities on a regular basis. Maybe once or twice a year for that?
I would argue the east coast works much differently. The traffic patterns are not so much between NYC and DC, but most people moving in and out of dense areas. Much of the Acela is M-F commuter traffic.
If it actually worked like Japan (and China?) maybe? In Japan, if I want to go from Tokyo to Osaka, I just take a local train/subway to the station and buy a ticket for the next bullet train. I can get the tickets on my phone as I'm on the subway (or buy them at the machine at the station). There is always a train within 5-10 minutes. There are trains pretty much every 5 minutes, some are express (5 stops to Osaka), some less express (~12 stops), and some go all they way to Kyushu. The only time this isn't true is the 3-4 weeks a year when everyone travels and the trains are full. On those weeks you need to get tickets in advance.
The point being, it's so convienent I can decide to visit my friends for a picnic and come back the same day (yes I have done this) with no pre-planning. I have not pre-purchased a ticket in years.
I don't expect California to have trains every 5 to 10 minutes. I also expect they might fuck it up like Spain and require baggage inspection. I do expect that if they finish building it (I don't think they will) that by the time they do, Waymo and similar services will be ubiquitous and so it might actually be useful.
It makes sense in the east coast: traveling from the seat of government in DC to the seat of finance in NYC. But does california have that dynamic?