The original rail network was on par with airplanes in terms of relative improvement of speed of travel. The highway network did the same for travel to areas unlikely to ever get a rail connection. (There aren't that many sparsely populated countries with a rail connection to every town; even fairly dense countries don't manage this.)

In today's world, HSR fills a very specific market niche; planes are much faster for long distances due to higher speed and small requirements for land acquisition, and cars are just as competitive for short distances due to the last-mile problem. So I don't think it's reasonable to connect every possible city pair in the US with HSR, but rather it's much better to just build strong regional networks of HSR that work on their own. There's no reason to build a continent-spanning HSR network.