There’s only one way to figure it out - train on a properly licensed content and show them that.
Your line of reasoning sounds like “ah, we already won so your protest doesn’t matter anyway”, but did you already win actually? Do you really not need all their development to draw on the same level? Just show that.
I'm not in AI and my GPU barely runs games from 10 years ago, so I'll pass. To be more precise, though, I think that it _seems_ that their protest won't matter, but the one way in which I see that it may (the second out of three options) leads to an outcome that I would just consider bad in the short term (for society, and for artists that are not established enough to benefit from any emerging redistribution system; we observe cases in Germany every so often where pseudonymous musicians are essentially forced to charge for their own performances and redirect proceeds to rent-seekers and musicians that are not them, because they can't prove ownership of their own work to GEMA's satisfaction).
Your line of reasoning sounds like “ah, we already won so your protest doesn’t matter anyway”, but did you already win actually? Do you really not need all their development to draw on the same level? Just show that.