You refusing to write open source will do nothing to slow the development of AI models - there's plenty of other training data in the world.

It will however reduce the positive impact your open source contributions have on the world to 0.

I don't understand the ethical framework for this decision at all.

Yes — That’s the bath water. The baby is the all the communal good that has come from FLOSS.

surely that cat's out of the bag by now; and it's too late to make an active difference by boycotting the production of more public(ly indexed) code?

If we end up with only proprietary software we are the one who lose

open source code is a miniscule fraction of the training data

It is. If not you, other people will write their code, maybe of worse quality, and the parasites will train on this. And you cannot forbid other people to write open source software.

Free software has always been about standing on the shoulders of giants.

I see this as doing so at scale and thus giving up on its inherent value is most definitely throwing the baby out with the bathwater.