The existing system would stay the same. A USD stablecoin would be something new - a cryptocurrency that is backed 1:1 by the USD and the US Government.
It's too big of a job for private finances and too prone to corruption because there is zero profit if you do it properly.
But why should the US government build something just so crypto folks have something to gamble with? Why does the government need to enable their habit? What benefit is there to its people?
What value does "cryptocurrency" add to a centralized currency? I can see that we'd be better off with a standardized protocol for sending money around, but plenty of countries have done that fine. The US is stuck in a technological backwater, yes, but I fail to see why "cryptocurrency" would decrease the number of problems.
It's too big of a job for private finances and too prone to corruption because there is zero profit if you do it properly.