It's already possible to add new comments to already closed issues, but IMO it's a design flaw that these comments cannot be edited or even deleted.

It's a design flaw, IMO, because for the use-case where you want to update some info/comment in an old comment, you are now forced to add another comment (rather than update ur existing one). So for this scenario we are back to pre 3.x, where editing comments was not possible at all.

Another case is: As an admin, you can't delete some inappropriate comments (like spam) without having to reopen the comment first.