But I am counting on the fact that _companies_ would be too worried about legal uncertainty to try this.
Oh, my sweet summery child :-)
Companies have been doing far more dodgy things, things that look objectively illegal where-as the clean-room reverse engineering thing looks to me (caveat: not a legal expert at all) to be legally defensible, for a long time, definitely pre-AI.
Even if a company operating in your jurisdiction, where you as an individual might have a cat in hell's chance of even trying to sort the situation to your preference, might not, you can bet your last <currency unit> that a company elsewhere will feel more free to do so if they wanted what you have without agreeing to your terms.
If it’s a small company, it’s not worth it, but then again they are not causing a lot of damage either. If it’s a big company, they will find me unexpectedly resistant to usual legal bullying tactics. I have legal insurance, multiple lawyers as friends and don’t have a job so have loads of time for filing paperwork.