If the AI writing is good, you’re not going to know it’s written by AI and you’ll continue to think you "can always tell" while more and more of what you read isn’t written by humans.
I agree with you that the "portion of written content online that’s AI generated with no obvious tells" is "small yet growing". That's exactly the thing - it's still too small to "be there yet" :)
E.g. it's perfectly possible that in terms of prevalence "AI slop > AI acceptable > human acceptable" instead "AI slop > human acceptable > AI acceptable" and nothing noted explains why it is one instead of the other.
Like imagine the Rust Evangelic Task Force, but for the next big thing, will probably be shilled by bots.
Low quality content like Youtube and Reddit comments are probably mostly LLM bots who comment on anything to hide the actual spam comments.