>Because a comment made on a public worldwide-readable forum is not private, and you know that before you sign up with an account, so it's not like you accidentally made a public comment that was meant to be private. Protecting private communication is about just that - stuff that actually is private (to begin with).
That's an incredible strawman. Nothing you argue is a reason why we can't delete posts with our accounts.
>How does a forum look like when key posts are suddenly missing?
I guess that depends where your values are: privacy or making sure a forum "looks good".