No one is using Midjourney to replace artists. Artists are using Midjourney to speed up the most tedious parts of their work.
If I can make a voice over that sounds like Neil DeGrasse Tyson, why would I pay him to make the voice over for me? If I can make artwork in the style of X why would I commission X to make that artwork instead? Nobody cares if it actually was made by them as long as people think it was made by them. So "them" gets screwed because nobody will use them. We're obviously talking in very strict ones and zeros here with "nobody". Except for the nobody caring what you think you sound like
All content will be behind a subscription and the algorithm will say "you like Janis Joplin? Let's autoplay some close-enough AI artists that don't exist that we won't have to pay any royalties to!" You'll get an offer to upgrade to Double Pro+ for a special introductory rate to skip those tracks.
This discussion makes people immediately place themselves under the boot of technology and display active helplessness almost entirely because they can't handle the mentality that it's now a commodity to get what they consider a "unique artstyle". That you can be "unique" if you consistently draw a line in thicker shades around borders and use flashy colors in a way no one has decided to try yet, and that this style "belongs" to you being extrapolated to the point of your artistic hobby becoming the only reflection of your identity as a person. Sad for those who staked their lives around capitalistic ideals.
That said it is unclear if the connection is strong enough between a given artist and their replacement.
My children's children are going to have a few classic books that were illustrated by humans, but anything new or reissued won't be.
Magazines, articles, and covers that might have featured human illustration will be gone. It'll be too easy to throw pennies for "draw something topical to article title in the style of artist that appeals to target demographic".