If you'd spend any time at all looking at midjourney creations, there's no way you can conclude that there's any significant amount of "ripping off" people's work. Certainly not in all or most cases.
With the right prompt you might be able to get something that looks eerily similar to an existing work, or artist style, but hardly anyone is interested in doing that, except for memes which are by definition modified and transformative. Nobody wants to AI-generate a mimic of a copyrighted work. If they wanted to do that, they'd just rip off the original work.
None of this is a new issue. The Warhol case even resolved one remaining facet. If you take an original work and apply photoshop filters to it, or do the equivalent manually or with AI, that doesn't alleviate the copyright compliance burden. The fuzzy area is how similar something has to be to fall under copyright; that'll remain subjective until the law or courts resolve it or copyright collapses entirely under its own weight, but the vast majority of AI generated content is nowhere close.