You're not taking into account the difference between theory and reality.
"In theory" the US government "cannot" due to technical aspects like the RISC-V Foundation being based in Switzerland.
"In reality" though, the US government could (for example) introduce targetted sanctions on companies having anything further to do with RISC-V.
The vast majority of those companies would terminate their RISC-V involvement on the spot, as they have substantial US involvement with their businesses.
The remaining ones could indeed keep on pursing RISC-V, but the whole "take over the world" thing would be either dead or have its timeframe pushed out by many years.
Note - I'm trying to illustrate that "inevitable" isn't correct when powerful players are involved. So the above example is just the first one to spring to mind.
The momentum has grown far beyond the point of no return.
As we can see it already happening, it is far more likely for me to be struck by lightning than RISC-V not take over the world.
>If RISC-V does make solid inroads into servers and supercomputers
See recent talk from BSC's Mateo Valero[0]. EU's supercomputer roadmap already is RISC-V centric, and their researchers already have a few generations of finished RISC-V designs.
>then (perhaps ironically?) the US gov may start doing funky things to suppress it
They cannot. RISC-V Foundation is based in Switzerland.
0. https://www.youtube.com/@tenstorrentinc (it's somewhere in the recent "Nerds Talking to Nerds About RISC V", which has a load of good talks)