You're right that the parent was a good contribution and deserved to be unkilled. That's what vouching is for, and you made an excellent use of it.

Banning on HN isn't punitive, it's preventative. When a banned account has changed its ways and has reliably been sticking to HN's rules for a while now, we're happy to unban it. If people see cases like that, they should email [email protected] so we can look into it. That's a fine way to watch out for a fellow user.

When a banned account hasn't been reliably sticking to HN's rules for long enough, or hasn't posted enough to decide that yet, it stays banned for obvious reasons. In that case their good posts can still be unkilled, just on a comment-by-comment basis. That's why we created vouching and it works well.