I don't think it really matters whether or not Midjourney themselves are liable, the output of their model being legally radioactive would break their business model either way. They make money by charging users for commercial-use rights to their generations, but a judgement that the generations are uncopyrightable or outright infringing on others copyright would make it effectively useless for the kinds of users who want commercial-use rights.
I wouldn't loose sleep over this if I was working for Midjourney. Copyright lobby is powerful, and when bad actors like Microsoft, Disney etc. jump onto the AI bandwagon and put their legal weight on their side of the lever, everything will turn out well (for them).
