That's over 50,000 people per hour so why does 50,000 per day seem unlikely?
Because neither LA nor SF are anywhere near as big and economically important as Tokyo and Osaka? (Among other things, Tokyo is the Japan's capital.)
That's being said, between the parent and grandparent I'm not sure who is right and who is wrong, there's a lot of flights and cars between SF and LA on a daily basis. Whether rail can take grow to absorb many of those depends on a ton of factors - such as travel time and convenience. SF and LA are big places, and a rail connection between them is only one link in the chain. You also need useful local transport to destinations within the metro area. (And even so, I suspect it'll take years to decades for people to get in the habit of using trains rather than defaulting to the airlines or the highways.)
But the main point here is one of the reasons why US rail (particularly passenger rail) developed differently from other countries - not just the sheer physical size of the country, but that we don't have One Big City to (all but) Rule Them All. Japan has Tokyo, England has London, Germany had Berlin, then Bonn, and now Berlin again, and the pattern repeats across the globe... one Big City that is the heart of the nation's government, business, and financial structures. One Big City that serves as a nexus for the transportation system. New York City once came very, very close... but even then it shared primacy with Chicago and Pittsburgh and Philadelphia. Even now it's probably the closest we (the US) have, but it still shares primacy with half a dozen cities scattered across the continent.
Thus the US is far better served by a series of regional HSR networks than by One Big Network, with the airlines serving to cross the gaps and the continent.
Setting aside the fact that most US rail advocates seem unaware of the various levels in a rail network - the locals, the limited, the express. They mostly seem to want to have their cake and to eat it too - high average speed, _and_ no city left behind. You can't do this with a single level network, and nobody even tries.