The difference is that as far as fanfic goes you cannot escape the fact that they aren’t real, it’s static text on a screen, and even people roleplaying are liable to get a “dude wtf” response if they have no notion of what are appropriate expectations and behavior. But an AI will go along with whatever it’s told to and try to appear like person doing it. They will validate and reward even the user’s wildest expectations.

If there’s people so lost in their fantasies that they will convince themselves they are in love with some scripted basic visual novel character, imagine what AI bots will do to them.

To be fair I don’t think this is downfall of society material, but I think it’s a given some people will go absolutely nuts because of them, and it might affect how they treat real human beings around them. The internet has enough unhinged people even when they are capable of interacting with each other. Imagine when we are dealing with people whose main practice of conversation is getting sexted by AI bots they treat like trash?