This suggestion is a fantasy and would imply leaving the Berne Convention and TRIPS ( Agreement on Trade-Related Aspects of Intellectual Property Rights ) which is part of the WTO.
Doing this only in the US would put our intellectual property in similar company as North Korea, South Sudan, and Turkmenistan.
You're right about it being a fantasy but not because the WTO has the power to overrule the US. It's because the political class of the US put those clauses in TRIPS for the benefit of the US people in aggregate - i.e. the "we're better off with draconian copyright law if it keeps China on a leash" argument that is implied when people complaining about "China stealing our IP".
If the US actually backed out of TRIPS you would see a large number of countries either following or being amenable to following them out. Most countries are net-payers into the current copyright system, which is designed to leash everyone else to rich countries, and would love not having to pay into it anymore. The biggest hold-outs would be the EU and Japan. It's important to note that the last copyright term extension was just as much Germany's fault as it was Disney's and the EU could easily propagandize[0] against a low-copyright or copyright-free US. And Japan is run by a right-wing corporatocracy called the LDP that constantly wins because nobody in Japan actually votes[1].
[0] The reason why it's easy is long and complicated, but basically, resentment of America goes back centuries and is a weird meme of the monarchy that cross-pollinated into every other political ideology of Europe.
[1] Which, to be fair, also applies to America. The difference is that the LDP's opposition is the Japanese Communist Party - which means the LDP was an American puppet while the JCP was a Soviet one.
> we're better off with draconian copyright law if it keeps China on a leash
The irony of this argument is that China is seemingly undeterred by any of these treaties. As long as China is the manufacturing hub, they can do what they want anyway.
We're so far down the absurdly long copyright duration nonsense that any proposition of a reasonable duration is labelled as "fantasy" and compared to north Korea, that's a bit scary.
Doing this only in the US would put our intellectual property in similar company as North Korea, South Sudan, and Turkmenistan.