How to win with delaying tactics
Suppose some politician started pushing this idea now with the intention of getting a law in place in a couple of years. If Big Tech put some effort into it I expect they could push out the date when it finally takes effect at least a decade, probably more. Then there'd be enforcement and the courts, and assorted appeals; I doubt it'd take less than 20 years to force ChatGPT 4 to become open source if it even succeeded.
At which point if LLMs are still big business their owners would 1) have replaced the old ChatGPT 4 with something else, 2) could have moved any affected money or operations to somewhere else in the world with more appealing laws, 3) IBG YBG. Sounds like a pyrrhic victory at best.