And I think the reason that AO3 made this decision is obvious: there's actually such a thing as "bad" tagging.
I agree: there may be no single "good" way of tagging, but that doesn't mean there aren't bad ways. The fic with thousands of tags was obviously bad because it broke the site, as well as the accessibility issues you mention. I would say that inaccurate tags are also bad, because they provide the wrong information.
Under-tagging is more complicated though. Authors don't technically have to tag anything beyond AO3's mandatory content warnings. Anything beyond that depends on what the author is trying to achieve with their tags.
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I agree: there may be no single "good" way of tagging, but that doesn't mean there aren't bad ways. The fic with thousands of tags was obviously bad because it broke the site, as well as the accessibility issues you mention. I would say that inaccurate tags are also bad, because they provide the wrong information.
Under-tagging is more complicated though. Authors don't technically have to tag anything beyond AO3's mandatory content warnings. Anything beyond that depends on what the author is trying to achieve with their tags.