I should've clarified that these main classes include sub classes. Wizard gets its various different sub classes, druids their sub classes, etc. Paladin and Ranger would both be under the fighter sub class.Fighters are non casters.
Clerics are casters that you use religious based magic.
Wizards use Arcane magic.
Druids use nature magic.
Bards use magic/magic.
Rogues have their own non-combat skills that are worth differentiating from fighter.
So what? Just use feats like arcane-magic, religious-magic, etc for the caster class and make them exclusive. And rogues can just be treated as fighters who picked up some non-combat skills over combat skills. Some PnP games, like modern editions of the The Dark Eye actually do something like this.
Also, how would Paladins and Rangers fit into your class list?
And I personally think that a classless system akin to Underrail's would be best, but I don't know how that would translate to PnP and D&D is known for classes so I think it's fine to keep their current setup.
Forgotten Realms as a setting is far from the greatest setting, and the sheer abundance of Wizards and magic users is a big symptom of this. One thing that might help is making Wizards require more experience than other classes. That being said, as someone else pointed out, balance isn't everything, and I'm fine with Wizards being more powerful and versatile compared to other classes as long as some tweaking is done to even it out just a tiny bit. I also wouldn't be against some more strict subclasses that separated utility and combat spells.The game doesn't need less classes, it needs more classes. The problem with the wizard that it's the class that can do everything, since his spell list is so bloated, and in the minds of WotC every spell makes total sense as a wizard spell. 3.5 had the same problem, but it also offered a solution by introducing three classes - Warmage, Beguiler and Dread Necromancer. Each of them excelled in their area of specialization, while also being bad outside of their area of specialziation.
Also, if any of you asks "But why is the class that can do everything in the game a problem?", don't ever talk to me again, you fucking retard