I'm glad you bought BG3 and all, but if you hate D&D, why did you buy a D&D game? I mean, why are you playing a D&D game and then upset that they made it based on D&D rules?
Support and mages classes do magic, but they also need to swing weapons and shoot with crossbows. They differ from archer or warrior in the gameplay because they aren't as good at these things AND their magic makes them more potent when handling bigger enemies. As you gain levels and become more powerful, they become VERY powerful especially as AOE and Damage Mongers. They need to be limited so that they aren't overpowering the rest of the classes.
You know what's fun? I never knew this was a DnD game. Unlike the others, I didn't follow the game, I was just looking for a new RPG, I had played DOS2 before, but I didn't know that Larian had a new project. When I found out, I immediately tried it and liked it. At that time, I did not read interview or detailed description of game.
I love RPGs, unfortunately (for me) many rpg games (like PoE1) use DnD system for skills. I don't drop games because just cuz they use this system, I say I can accept it, but I will never consider it as plus. This makes their gameplay similar to that of other classes. Yes, of course, you have "normal" skills that do not need slots, but, for example, Cleric skills at this moment in BG3 are simply terrible. I don't see the point of using them, because it's much more profitable to shoot a bow. This makes my experience on cleric exactly the same as my experience on ranger. I just shoot bow until there's a boss or something like that. What I really love about DOS2 is that every battle you fight (unless you're a barrel fan) is magical, literally.
But like I said, I accept it. Because I love story and role-playing moments more than gameplay.