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.
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.
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.