I do think there are some odd unhealthy things that go on in fandoms now, not just HP by a long strecch.
I don't think it's quite the same as sports, though there is some unhealthy stuff with some sports fans too.
In my youth I was quite involved in role-playing games, and also with the Society for Creative Anachronism, so it's not that I'm really against roll-playing. And I was also involved a bit in some SHerlockian societies, so I am ok with some silly fun literary analysis and speculation.
It seems instructive to me though to think about the Sherlockians, because they were weird, but nothing like the toxic fandoms I see now. For one thing they seemed a lot less likely to be fleeced by junky tat sellers. But also - there was always a clear awareness that there was a level of pretend, it was self-aware, and there was a lot of humour in it.
Fandoms now seem so serious, and often toxic, and like they are way too much a part of the self-identity of the people involved.
Sometimes it almost seems to me it becomes a sort of substitute religion. (I went to a DR Who themed wedding once where that seemed particularly clear.)