I would agree that signature ban is better than permaban.
Signature bans are only allocated to deserved ones who have good net-effects. Not all of permanent banned users will get signature bans as alternatives.
Earning merits is not easy, so they have to prove and write better posts to recover their signature space.
If one permanent banned user received an alternative as signature ban, that one does not have to prove anything to get signature space back. When the signature ban period expires, signature space will automatically reappear.
The thing that user has to prove is good net-effects to receive a signature ban.
Permaban in most cases results in members just creating new accounts that are doing exact same plagiarism.
If one user put decent efforts for one account, and get permanently banned because of stupid things (mostly plagiarism) in the past, in early days, ie. I don't think that one will keep doing the same type of stupid things again.