Basically the title.
I have seen the EU-OS/Suse discussions for some months now. However, Ubuntu/Arch/Fedora are extremely mature projects. So competing against them will be hard.
I want to know how realistic the scenario (described by the question) is.
Canonical? the US could try but Canonical isn’t a US company so far as I know. The attempt would probably just piss off their “home” nation. That would be the UK, I think.
Red Hat is another story though. It’s owned by IBM which is a US company, which means it is, in theory, obliged to obey any lawful order of the US government. I say “in theory” because there is a long history of companies here saying “Yes sir, Yes sir, Three bags full sir.” and then doing whatever they want when no one is looking anymore. For examples see Facebook, Google, OpenAI, Exxon IBM, Coke, Ford and… Well just about every company that has been around for more than 20 years and most small businesses to boot.
Practically speaking, though. These companies are based around open source projects whose source code has been widely distributed. If you need to, (or hell, even if you just want to) fork them, rename the project to avoid trademarks, and move on. Whether you flip Uncle Sam the bird as you do so, your call.
There’s also the fact that a legal order could be seen as violating the First Amendment, and I make no illusion that the Administration will ignore it (like the rest of the Constitution) but barring a bribe, IBM or anyone impacted could at least go to the courts and get a slam dunk injunction for some period of time.