You might be able to replace some of the long-distance Amtrak lines with such a system, but over half of Amtrak's ridership is on the Northeast Corridor between Boston and Washington. Those trains make stops at all the major cities (and some minor ones) along the way. This is why the "Acela Express" is high speed in theory only. In order to keep the same ridership, any new train would have to do the same thing.
Yeah, replacing all trains is impractical in densely-populated areas that are already well-suited to classic railways, but Hyperloop would make it possible to see similar numbers riding to and from central states.
Also, I bet NYC-Washington or NYC-Boston hyperloops would pay for themselves in two weeks.
You don't seem to be taking into account that a much faster and cheaper train would vastly increase the ridership between any two points. If it was $30 and half an hour to downtown LA, I would visit my brother down there every other week. As it is, we see each other every few months.
