10 hours is still significantly faster than a hypothetical HSR. Remember that it would also have to stop along the way. Secon, not everyone lives/is traveling to a place that is near a hypothetical train station; you have to keep in mind that American cities tend toward sprawl. HSR leaving from e.g. downtown LA would still be less convenient for e.g. the folks who live in the Valley. It's definitely not a 10-hour excursion for many people to transit the country (source: I did this for years for work).
> Also you waste a day or ruin a night's sleep on the redeye.
Highly dependent on the individual & service class. But the point of this is you're actually physically there in less time.
> Leave LA tonight, shop/sightsee/eat all day in NYC, take in a show, hop on the late train back to LA the following day. This is pretty near impossible today unless you have a private jet.
You're going to get a lot of pushback from people who have been to weddings on the other side of the continent, since they will have done this. Most of them do not have private jets, it's just a common thing Americans do.
Keep in mind these old downtown stations built in 1900 won’t be able to handle the kind of passenger counts that would “need” to be shifted from the airports, so don’t delude yourself into thinking you wouldn’t need big train stations built in the outskirts, crowded and irritating to get to, like we have with modern airports.
Your day trip thing sounds even more confusing. That itinerary that’s “near impossible” with fast flight becomes more impossible with trains that take 2-3x longer. Also, sleeper train tickets are incredibly expensive compared to coach, because of how much space they take up and the cost of upkeep for these little suites. They’re always going to be expensive, like the redeye flight on a lie-flat bed. So again, no different than what we have today, just much, much slower and less accessible to the masses.