If I'm deleting my account there's a reason for doing so. I don't want the company I'm trying to leave to continue to hold on to my data for any reason whatsoever. If I did, I'd just cancel or deactivate my account. There are often legal reasons for doing this as well, particularly when handling PII.
Incidentally, this is why I not fond of engineering solutions like append-only databases. They deliberately take agency away from your users/customers in order to make life for the engineers easier.
Then we could deal with that by adding another link that says 'if you really want to delete your stuff right now without recourse then click here and it will be done'.
This of course would cause a ton of trouble for those cases where someone managed an account takeover or for people who are really clueless. See upthread comment about the number of CS calls such implementation details can lead to.
The shorter version: you can't please everybody. There will always be exceptions.
Incidentally, this is why I not fond of engineering solutions like append-only databases. They deliberately take agency away from your users/customers in order to make life for the engineers easier.