No, it's not. Numbers and the manipulation there of is a very parochial mechanic to use and results in a narrow style of play. Numbers will result in players being forced down certain roads because it's the path of least resistance, changing mechanics and intelligence allows characters to play how they want..but against smarter opponents.
Right now (good) players have to avoid many tactics to get a decent challenge. Reducing the effectiveness of a small number number of overly strong skills won't force players down one road, but will actually incentive them to use other skills that were just inferior before. I agree that AI and mechanics are the most important things to fix, but number balance has its place too. I guess we'll just agree to disagree.
The early part of this thread was a discussion of just how easy the game is. Except for the beginning and a couple boss fights, the game is really quite easy. Fixing numbers will take us from easy to "I actually have to think a little bit since the 'I-win-automatically' tactics will be gone," and then fixing AI (by increasing variety of their behaviors and reducing their dumb mistakes) and changing some mechanics will actually make the game hard in a fun way. Changing numbers can create tedium if fights become formulaic and long but not challenging, so Larian certainly has to be careful here.
Is variety part of the answer? Most certainly, but variety is also already part of the problem. Players can access basically everything and they don't make very many sacrifices to do so. Mages don't have to specialize, and many great spells or abilities in all schools are available with one point in a school. There should be many more spells available overall, but players shouldn't be able to access quite as many of them. There's probably nothing Larian can do about that now, though. I don't think there's a simple, non-radical solution to the classless system. The best we can do is to rebalance a lot of the skill trees so they're not so frontloaded with amazing spells. E.g., you shouldn't be able to get a 3-AP oath of desecration with 1 point in Witchcraft.
On the one hand, I want variety and to be able to use it. On the other, I want to have to make serious choices about spells and abilities. Hopefully adding a lot more spells for each class will help both of those, but I think there'd have to be deeper changes to really find that sweet spot.