Basically the fewer full spell casters you have, the fewer types spells your game can support.
It's why the conjure spells are being nerfed. Caster turns were too long and it could be often.
Why combining Paladin and Cleric spells into one Divine list didn't work. Clerics with smites.
Druids with ranger buffs. Nope.
You could have
2 casters
4 casters
6 casters
10 casters
But there are story and gameplay ramifications
It's why the conjure spells are being nerfed. Caster turns were too long and it could be often.
Why combining Paladin and Cleric spells into one Divine list didn't work. Clerics with smites.
Druids with ranger buffs. Nope.
You could have
2 casters
4 casters
6 casters
10 casters
But there are story and gameplay ramifications