> Perhaps this illustrates a fissure that was always lurking under the surface, then(...)
Yes, I do think there has always been such a fissure. People publish OSS code for many reasons, often a blend of multiple reasons. There are selfish reasons such as the desire for one's work to be recognized, or even the hope of getting better employment through showing ones' skill or making something companies will pay for support on. There are social reasons like the desire to collaborate with others. There are altruistic benefit-of-all-mankind reasons like Richard Stallman said "...restrictions reduce the amount and the ways that the program can be used. This reduces the amount of wealth that humanity derives from the program."
It sounds like your view of things is limited mostly to that last version of FOSS, the copyleft style. But even adherents of that style, I think, are not too happy with AI consumption of their code. For one, it allows laundering of the copyleft license so their work goes into closed-source products that are never shared. And for two, if your idea of OSS is that we all put our contributions into the great shared river of human achievements to benefit the world, it is disappointing to see that river funneled into a giant waterwheel of profit for a half dozen trillion dollar companies charging rent for its bounty.
> Given that FOSS licenses were always constructed to function within applicable copyright law, I don't see how they could mean anything else.
I agree from a legal standpoint. I cannot enforce my personal definition of copying nor do I expect that to become possible. It was just conveniently aligned with the reality of how copying software worked in the past, and no longer is and never will be again. That doesn't mean I will be writing OSS software with a new made-up unenforceable license. It just means, like OP, I'll weigh differently whether I want to bother releasing stuff at all.