The understanding you're missing is that artists benefit when they are credited for their work. Let's say you drew concept art for a videogame, and your name appears in the credits. You can now go to recruiters and hiring managers and say "I worked on a game that shipped" and get hired.

Let's say, during hard times when you don't have a job, fans of the game you worked on notice you on twitter and say "hey I loved your work, here's $5 for your patreon." Now you can pay your medical bills this month.

Their livelihood was taken from them. Midjourney is theft.

Nothing except their ability to make a living.

Value has been taken. Motivation to create has been taken if the second you upload something it gets sucked in to a model that any old person can create infinite iterations.