Fundamentally, I don't actually care about whether AI wrote it or not. I care about whether the ideas are well-thought-out or half-baked. I care whether the writing is slop (whether AI or corporate PR or politician-speak or whatever), or whether the writing is actually something a human would want to read.

But as you say, we don't know how to write a checker for that...

I fully agree with you that whether AI wrote it or not isn't the actual issue.

Luckily, it is fundamentally easier to measure the quality of the writing and argument than it is to determine whether a human wrote a text or not in the face of motivated "attackers" with access to whichever judging mechanism you use.

The problem will be for the people who do care whether AI wrote something or not rather than about the quality.