I heard a bunch of explanations but most of them seem emotional and aggressive, and while I respect that this is an emotional subject, I can’t really understand opinions that boil down to “theft” and are aggressive about it.

Why the anger?

How do you earn a living yourself? Or even better, what is your most precious hobby? Whatever it is that you love doing for the love of it (that’s the definition of a hobby) try imagining being told one day, out of the blue: ‘Guys, my fancy but completely soulless computer can do as good as many of you. And it can do it in seconds. Wanna compete?’

Now, imagine it’s your job and not your hobby, the way you earn your living (and pay your rent/mortgage and those always more expensive bills) and imagine being told 'That way you used to earn a living? It’s gone now. It instantly vanished in a magical cloud of 1 and 0s. This AI-thing can do in mere seconds something that would take you weeks and it can do it well enough that quite many of your customers may not want to spend (a lot more) money to pay you for doing the exact same job even if you do it much better. How happy would you feel about that?

So, yeah, like you said it’s kinda ‘emotional’ topic…

is there an argument against models that were legally trained?

Being 100% sure there exists such a database that contains no stolen creation, and then that AIs were indeed restricted to it for their training is already something worth debating and doubting (the second it is not open source), imho.

There had been a similar problem a few centuries ago, when photography first appeared many painters rightfully considered photography a threat to their business model as one could have their portrait (edit: or have a picture of a landscape) made in mere minutes (it was a longer than that, early days photography was far from being as quick as we know it but you get the idea).

What happened to them and their practice?

1. Some painters had to find rich sponsors that were OK to pay in order to get a portrait that would be more unique than a pĥotography (I know what I would prefer between having my photo taken by even a decent photographer or, say, a painted portrait made by Sargent), others found niche domains were to could still earn a living, while others simply went out of business.
2. Others decided painting could be much more than just being realistic like it (mostly) was before photography became a thing and they quickly started offering us amazing new kind of paintings (impressionism, abstract painting, cubism, expressionism,…)

And here we are in the XXI century. Painting is still doing fine in its own way (exposed in art galleries and in the home of rich people). There is also a lot more hobbyist painters that will paint all they can including realistic scenes no matter how much ‘better’ a photo could be. They don’t care. Next to those, there are many photographers taking countless photos (many of which being worthless too), some of them trying (and many failing) to earn a living selling them.

is it something past the saying that AI art is lifeless?

Maybe it will get better, most probably it will, but so far I feel real sad for people that are unable to see, to feel and to understand how lifeless and how clueless AI art is.

Edit: typos (yeah, this was handwritten without the help of any AI :p)