I would also prefer if IDs were never removed. And with comments (including comments in IDs), at the very least have the comments remain but delete the text and replace it with something saying the user left. Then people can tell that there used to be a comment there.
It should definitely be possible to remove all your observations, but I don’t think it should be the default.
I just read through the Google Group discussion and I really like the suggestions by Ben Phalan, particularly the ability to remove media and username from all observations but leave the taxon records and discussions (meaning they will be casual). @jonathan142’s suggestions were similar.