The big problem is false positives—human-written text that gets incorrectly flagged as being written by AI. There's going to be so many false positives in any scheme like this, it'll barely be worth it.
You also have to remember the future of AI is not just LLMs getting larger and larger. There will be something after them, and something after that. I'd guess we're maybe 2-4 years away from an AI that hooks up to an LLM but has "actual" knowledge of things so it doesn't confabulate new facts, which would remove one of the major signals I'm currently looking for in GPT content.
Expect lots of false positives and honest businesses being affected. How much should the filter threshold be tuned to smash more bots instead of real people?