But at the point people stop using general purpose search engines because there's so much content spam that they're unable to distinguish from genuine material,

But at the point people stop using general purpose search engines because there's so much content spam that they're unable to distinguish from genuine material, then there's no further motivation for anyone to keep the content spam bots going, until the search engines' own SEO-detection AI catches up. Some sort of arms-race is likely inevitable, but I'm not convinced GPT-based tech is going to especially favour one side over the other.