For a model trained exclusively on the public domain, I would still be unhappy about the prospect of a world where a lot of low-level gigs that keep artists alive have been replaced by spending a half hour fucking with prompts. It would however be free of the "this corporation abused fair use to create a training model which it is selling to people" problem.

And no, it's not infringement if you work in someone's style - but it's also not professional to do this for too long, especially with a living artist. One piece in someone else's style is a homage, several years of pieces in their style starts to get you labeled a "clone" or a "style thief". Unless you are their assistant, in which case "working in their style" is exactly what you are being paid to do.

If you would like an example of this happening, read the 'controversy' section of Kieth Giffen's Wikipedia page, which sketches out the way fans and fellow artists took Kieth Giffen to task for doing a complete style swipe from José Muñoz' shadow-drenched crime comics for several years: https://en.wikipedia.org/wiki/Keith_Giffen#Controversy