I don't think that a 10x estimate is credible. If it was I'd understand the ethical argument being made here, but I'm confident that excluding one person's open source code from training has an infinitesimally small impact on the abilities of the resulting model.

For your fire example, there's a difference between being Prometheus teaching humans to use fire compared to being a random villager who adds a twig to an existing campfire. I'd say the open source contributions example here is more the latter than the former.

Your argument applies to everything that requires a mass movement to change. Why do anything about the climate? Why do anything about civil rights? Why do anything about poverty? Why try to make any change? I'm just one person. Anything I could do couldn't possibly have any effect. You know what, since all the powerful interests say it's good, it's a lot easier to jump on the bandwagon and act like it is. All of those people who disagree are just luddites anyways. And the luddites didn't even have a point right? They were just idiots who hates metallic devices for no reason at all.

The ethical issue is consent and normalisation: asking individuals to donate to a system they believe is undermining their livelihood and the commons they depend on, while the amplified value is captured somewhere else.

"It barely changes the model" is an engineering claim. It does not imply "therefore it may be taken without consent or compensation" (an ethical claim) nor "there it has no meaningful impact on the contributor or their community" (moral claim).