It is absolutely 100% legal (and so it should be) for me to learn to paint by copying others' styles. In fact I'm fairly confident that the artists complaining here will have copied the styles of others in developing their own—perhaps even synthesising some of the ideas and blending them to create something distinctive.
But when you're able to do this at scale, and with no individual creative input (at least, beyond the prompt), it has the effect of devaluing the original art.
If I can type something like 'Dragon battle with a man at night in the style of Greg Rutkowski' and get the quality of output illustrated in the article, then how will Greg ever be able to sell his work? And if he can't sell it, why would he create it (or at least, why would he publish and market it) in the first place?
So unless we do something to protect these artists, then overall, creativity may decline, and we will end up looking at facsimiles of pictures created by artists a long time ago.
I'm not sure copyright is the answer, but I don't know what is.