It is more like you hire someone to repair a pipe and when they open the wall they find mold everywhere. You had been happy living in the house for the last 15 years, but now you know about the mold and you feel grossed out being in the house.
I feel like that's not a good analogy either; it's not like there was a flood of AI-generated garbage on these forums for the past fifteen years; it's not like a person discovering that they've been chatting exclusively with bots, not humans, for the past decade.
I think these forums are going to discover that a different, more stringent kind of moderation is needed now that doing this kind of shit is cheap. I guess maybe it's more like you've been living your house happily for 15 years, but with the change in climate, suddenly your house is full of a kind of rodent that hasn't been able to survive in your region, and you're just shoveling out buckets of rodent poop every day, and trying to figure out what kind of traps to set out to stop them from coming in.
(Except the rodents aren't just appearing naturally; your neighbor started a rodent breeding farm, and is releasing them into your house so they don't have to pay for extra food or space, and is selling them for a profit while your house gets slowly filled up with rat shit.)
Again you are assuming the people using ai here are barbarians invading this forum. I suggest they may be the same people that were there anyway, and have been frauds all along. Before they posted other people's work (taking pictures of other peoples work irl, buying things at thrift shops to post about, stealing pictures off other forums), now they are using ai to fabricate it. The use of AI did not lead to any new fraud in my scenario, it actually made the fraud obvious for the actual experts who were previously fooled.
It is more like you hire someone to repair a pipe and when they open the wall they find mold everywhere. You had been happy living in the house for the last 15 years, but now you know about the mold and you feel grossed out being in the house.