Well, I think the current distribution is far better. A mad mage who studied forbidden magic is a wizard, like all mages who study. A “mage who made a pact with an entity to get power or magic”, by definition, did not studied. A mage who studied and also made a pact can have both classes.
There is the pact of the tome that emphasise the idea that you can get a magical tome to get spells. You didn’t wrote those spells. You still didn’t learn this magic. What intelligence is there to this craft?