Well, if all the interstates did have plenty of intersections, and someone proposed removing them all retroactively, would that not be an absolutely gargantuan undertaking at present, to the point where justifying it would seem fanciful?
On the other hand, who says you have to do the entire country at once? Perfect is the enemy of good and all that.
There’s only 3 US states with a density higher than that (Massachusetts, Rhode Island, and New Jersey) and an additional 2 US states of at least 220 people per sqkm (Maryland and Connecticut).
The US isn’t population dense enough for such transportation to make sense.
I do tend to prefer rail whenever viable, and the cascades route is already halfway decent, just not competitive on any front except ease of access with flying.
People underestimate how spread out US cities are.
Tokyo has 8.9M people while LA has 3.9M people; half the number. Osaka (3rd largest) is 2.7M people while San Jose (3rd largest) is just under 1M people; again, half the number. The distance from Tokyo to Osaka is 500km, while from LA to San Jose is 550km. (In both cases, the 2nd largest city is nearby the largest.)
You have half the justification in CA you do in Japan, looking at similar urban areas. In the case of Japan, connecting those two cities goes through another 3 with over 1M population (and totaling over 7M together).
US cities are spread out and relatively low population.
The US historically had much more passenger rail than it currently does. You haven't provided any convincing reasons why that shouldn no longer be workable in the modern day.
Paris has 2M people and Bordeaux has 1M people (sixth largest urban area in France), they are 500 km apart. How come France is able to operate a dedicated high-speed line between these cities? The third largest city on that line is Tours with 360k people or so.
California has almost the same population density as France, actually, but it's less spread out so just one line would be much more useful than a single line is in France.
Canada just announced a high-speed line project between Québec, Montreal and Toronto, by the way.