Thing is, with abundant storage and good search, there's simply no reason to delete anything. Worst case, you'll never look at it again.

This. There’s zero cost to keeping them - including when migrating hosts! It wasn’t all that intentional, I just didn’t need to delete them.

Also, I wrote a lot of those emails (over 25+ years…). Sometimes it’s handy to refer to things I’ve written.

Maybe you use email very differently? I find myself occasionally needing years-old emails to find info I've long-since forgotten and it's a lot easier to have all of it in one place.