The illegal fork of the former Bitcoin codebase we're talking about here would certainly be tradable, but it would be vastly less liquid, and thus far less valuable.
The US government already controls the on and off ramps of most Bitcoin liquidity. I don't understand what the benefit is with attempting to "force" Bitcoin investors into buying a fork that might end up to be just another worthless one. From the government's perspective, since it surveils most high value Bitcoin transactions already, it stems to lose a lot more by meddling with the protocol, even if it were feasible in the first place. Changing the protocol is one of the most likely ways that Bitcoin fails, and if it does, the government loses the declared capital appreciation from all those bitcoin holdings.
Like I keep saying, the point of the OP is not
why a government like the USA would vote to change Bitcoin, or whether they might do that soon, but rather that it's completely possible and even plausible. There's nothing magic about Bitcoin that places it above the reach of laws.
All that said, to paint a possible scenario...
Imagine if the Major Players who collectively own a huge chunk of BTC and/or have custody of most of BTC figured out a scheme that would benefit themselves, and it required major changes to the Bitcoin core codebase. They would do what they've already been doing, and lobby Congress to pass a law saying that their proposed fork is "the real Bitcoin" and everything else is a fraud, and will be prosecuted as such.
And I've brought up the specific example of changing the architecture to a smaller, trusted set of nodes along with eliminating PoW, which would make the system scalable to handle all of the world's daily payments natively, which could vastly (as in, 100,000x) spread the usage of Bitcoin because everybody could use it for absolutely everything, not just investment. Hence the whales could believe their BTC would greatly increase in price, yielding them billions in gains--far outweighing the investment they made in political contributions. (Again, this is just a scenario that the Whales etc. might believe in, I'm not going to debate whether this would really expand BTC or not, etc.).
The combination of major holders shifting over, plus laws in (at least) the USA and probably lots of other places where the Whales would do the same lobbying, would instantly turn the new fork into the Bitcoin almost everybody uses, and the original fork akin to Monero.
Again, I'm not saying this will happen (although the more I write this the more likely it sounds!), but the point is that it absolutely
could happen, so let us please stop saying, "Bitcoin cannot be controlled by governments" because it absolutely can be.