The Valkyrie was wound down because missile technology made it obsolete before it was even done. The USA instead focused on the supersonic B1. And even that project was almost cancelled as obsolete if Reagan hadn’t had intervened, and gone for a low flying B1B instead that had much less chance of being shot out of the sky before dropping its bombs. To be honest, even the B1B isn’t that useful, ballistic and cruise missiles do the job much better. Even Russia doesn’t fly its supersonic bomber very often, flying the less expensive to operate TU-95 a lot more.

War isn’t a pissing contest to see who can have the fastest X, it is rather one where you find a bunch of most effective tools for the job.