Qualifying this statement by saying this is something I've thought about and this is my own bullshit theory.
There's a limited amount of "attention" on the internet that's available in online art spaces. I theorize that artists need to maintain a public portfolio to find work, either creative work for advertising or entertainment companies or commissions or patreon subscription. You need to be out there for people to hire you if you want to make a living doing art.
AI art is disruptive and harmful to that ecosystem in a few ways. It competes with artists not only for dollars and jobs for commercial art (ie ad agency work), but more fundamentally it competes with artists for attention on the internet, which at a macro scale makes it so there's less money in the system for real artists to practice and master their skills. It's also not necessarily that the AI art is better, it's just good enough to pollute the signal to noise ratio. And as you said, it's fast. That can quickly overwhelm sites like DeviantArt or Art Station, which moves those communities and resources away from their original purpose.
Additionally, AI art models need data from the commons to function. I'm skeptical that synthetic data is good enough to use for improving these models, so theoretically, one of the main ways to improve these models is getting more and better data from humans. If the economy of these models is structured in a way that discourages artists from posting their work publicly, or working at all, then the pace of improvement decreases. I think a lot of artists are pretty pissed off that their work is being used to produce commercial software that creates substitutes for their work without their permission or knowledge.
So that's what I mean by the tragedy of the commons. The online art ecosystem is more fragile than many people think it is and ai companies are over exploiting it. The content remains but the thing you're actually trying to sample, human knowledge and skill, withers on the vine.