Being someone with a foot in both worlds gives me a slightly robust viewpoint on this topic, so I try to chime in whenever I see this argument pop up. For reference, I have an MA in Visual Effects and a BS in Applied Mathematics, and work closely with artists and technologists in my job. I say this to support my credibility.

1. You are absolutely correct in who we should be mad at. Not the AI developers, many of whom are just trying to explore what is possible and make something cool, but the megacorps who are profiteering from the invention. All of the companies that are pushing AI as another SaaS and the ones who are trying to use it to replace artists instead of augment them. 1a. The other two specific groups we should be getting the torches and pitchforks for are the politicians who put so much legislation through that they circumvent our legal right to negotiate contracts we have to sign (EULAs in this case) and the companies and individuals who take advantage of our impotence to negotiate by placing abusive and abhorrent IP rights clauses in the contracts. To be 100% clear, when Deviant Art was scraped, nothing was stolen from the artists. They had all signed away the rights to their artwork when they uploaded it. The material was stolen from or provided by DA. They owned the rights, they owned the art, they were the ones who were ripped off.
2. “Ill-gotten gains” is a little strong of a terminology. At worst, it was dubiously obtained. The training of an AI is not that dissimilar to an artist looking at art they like and trying to recreate it to learn from the other artist, then attempting to make original pieces with what they learned. The only difference is scale. If you ask a practiced artist to recreate Water Lilies, if they have studied it and practiced Monet’s style, they would be able to recreate it with varying degrees of success. AI training is entirely destructive to the input material, nothing of the actual original survives, just an abstracted mathematical representation.
3. You are so close to right on what the rights of artists should be. It should definitely be opt-in, not out. When posting anything online, the displaying company should only be provided a license to display the material, not ownership or non/exclusive transfer of any rights. Any and all uses of submitted materials should need to be expressly and explicitly requested from the content owner without exception. The fact that Disney can sue an elementary school for self-writing and self-producing a Frozen musical for the kids but I cannot tell Facebook that they cannot use the artwork I post to a group in their advertising is asinine. If they want to use my art, they should be using their wonderful chat system to send me a message and asking me to sign a consent form to license the art.

All in all, I advise to avoid blaming the AI engineers (most of whom are altruistic in their motives) and the users (most of whom just want to have fun and play) and focus on the politicians and profiteers. They are the real villains in the story, and also the ones who seem to manage to stay under the radar.