The sooner anyone making profit from models trained on creators proprietary content start paying for the content they’re using the better for creators, society and even the AI companies. It’s pretty tiring hearing people argue about whether copyright law applies to AI companies or not. It applies. Just get on and sort out a proper licensing model.
It applies - but how it applies is simply not settled. Clearly, there is no statistical model that can launder IP. If someone uses Midjourney to produce someone else's IP, that user (not Midjourney) would be in violation of copyright if they use it in a way that does not conform to the license, such as by creating their own website of cat and girl comics and running Adsense on it.
However, if other artists are inspired by this style of comic, and it influences their work - that is simply fair use. If that artist is some rando using a tool like Midjourney - that is inspired by the art but doesn't reproduce it - it is not at all clear to me that this is not also fair use.