I have seen this argument before and it doesn't make sense even in theory.
I used to work at a company that did open source work and also proprietary work with third party closed source code. The company didn't let anyone who had seen proprietary code contribute to open source, because they felt once a person 'learned' from a proprietary codebase, then it's too risky if similar looking code lands in a project.
Imagine if someone saw the source code for Excel. Then sometime later they notice that Calc didn't have a feature that Excel did, and contributed an implementation of the feature. Even if they hadn't been looking directly at the Excel source code in the moment of implementation, you think Microsoft would be so "understanding" when they see someone that once worked on Excel contributing what could be construed as infringing?
The AI companies also seem to acknowledge this, as they have offerings that promise not to use your proprietary code as training fodder. If it is not a risk of infringement, then why would it matter to promise that the proprietary code is kept out of training data? Though it was short lived, why would OpenAI have even made a deal to license Disney material if it's all fair use anyway? If this sort of stuff is fair game, why do they get so pissy when other companies distill models?
Even as the AI company's have roughly defended this scenario, their defense should be a cause for concern for users. Generally they say that anything they do with things they can read is 'fair use', and when exhibits of clearly infringing outputs are given, they respond with the model only did that because the user's prompt directed it, and thus the responsibility for infringement should be with the AI user, not the engine that produced the infringing output. So the possibility of an unwitting infringement is possible as the AI companies explicitly say it's the fault of the user even if it happens.
But we come to your last point, that essentially at this point, the whole thing is 'too big to fail' and thus the practical risk is low. Which is true. It's just a bit disheartening that these companies are given free reign to interpret intellectual property law whichever way is convenient in the moment.
If the enthusiasts get interior speakers to vroom at them, fine. The ones where they point the speakers to the world though...
But ultimately, after the entire history of cars being "vroom means fast", I think it is a tough pill for them to swallow that it isn't a necessary facet of going fast.