That would be a completely radical rework to the entire balancing of the skills, economy, and Memory attribute, and it would not be possible to do on existing saved games (at least, not without making existing characters much more powerful and eliminating a gold sink). That kind of thing would be even more of a radical change than to the armor system.
It would not be easy to implement and test. I do not believe that Larian would be keen to upend the whole system like that.
My only suggestion would be to transition over time - keep some points into Poly but put a couple into necro and then gradually build up your library of Necromancer books over time and replace the Poly skills. You can respec the remaining points out of Poly later for free. (Turn off Light shadows in the Graphics options before respeccing, though!)
