I doubt the use case in the US is to link every major city. The air network does this far more efficiently. Places in the US are way too far apart.
High-speed trains would do well where conventional trains are too slow and regional jets are too expensive. (Example, Chicago-Madison).
I also see high-speed trains as being useful for city/airport links. Oslo has a high speed train between its central station and the airport, which effectively abstracts away the fact that the airport is in the Middle Of Fucking Nowhere. (Tokyo suffers from this problem, as well. While flying from HKG to NRT recently, the thought crossed my mind that landing in KIX and taking the train to downtown Tokyo might actually be faster...)
Even in cities like Chicago where the airport is close to downtown, conventional trains are too slow. (It takes me at least 45 minutes to get from Clark/Lake to O'Hare. A high-speed train could do that in 6 minutes! I would gladly pay a premium for this convenience.)
Finally, cost is not an issue as infrastructure is never profitable. The interstate system is not making money, the airlines are not making money, and conventional trains are not making money. Society needs the infrastructure regardless, which is why we pay taxes.