How about we give it some time, 90-120 days to let the temp bans expire and see how the sig-less copy-pasters perform. Will some of them disappear due to not getting paid?
If they disappear and return after their signature ban expired, it means they didn't deserve the lenience in the first place.
In which case the sig ban is good enough, as it gives good users a second chance and effectively bans the financially-motivated ones. Will they abuse the sig ban? Not sure how they would do that though.
Some avatars pay more than most signatures

There is no avatar ban yet, even banned users get to keep it.
So what safeguard from a person looking to hurt someone from doing this to an old post
That's a different problem, yes. There's this:

However, that is not nearly adequate. A moderator does not see what has changed.
Admins can see all old versions of edited posts.
A better way of framing someone would be to use an external site under your control, and fake the creation date of a post.
This has happened.