Amtrak's ridership has been growing and hit an all time high. The Northeast Corridor is a profitable route which is held back by century old infrastructure. If it actually had real high speed spanning the whole route it's ridership absolutely go up. Moreover your comments on not wanting to go from city center to city center are very off. Going from Union Station DC to Penn Station Philly, or NYC is really nice. Have you ever rode Amtrak, specifically that route?
The ridership and convenience of the Acela route is really not representative of the rest of the country.
It's extremely speculative to extrapolate that dense route with the induced demand for travelers going from Kansas City union station to Denver union station... with absolutely nothing in-between.
You don't need to cover the entire country. Building out routes that are already seeing demand would be plenty.
Yes, that is, to within a rounding error, Acela.
The new Borealis route (short run of the Empire Builder from Chicago to St. Paul) was profitable in like, weeks, after it started running. It is consistently sold out.