> But if AI is detected in the phrasing and not disclosed, it begs a lot of questions.

absolutely zero questions from me. If I see two exactly same writings: one - by human, another - by AI. For me its doesn't matter.

> Most longform readers will assume an author has deep expertise and spent a lot of time organizing their thoughts, which lends their ideas some legitimacy and trust.

It's the incorrect assumption of "most readers". Before AI there are enough methods to throw a long read. So, AI isn't really a gamechanger here