Again, thinking about 5E spell casters.
Classes exist in the game to allow players to do different things. In OD&D, Fighters fight. MUs use magic. Clerics do a little of both. Yet even here, MU spells and Cleric spells are not the same. Cleric spells help us. MU spells hurt them. OK both have utility spells and some of the other's main type. But to break it down simply, the analogy works.
In 5E we get Bards, Clerics and Druids whose spells on the whole "help us" and Sorcerers, Warlocks and Wizards whose spells mostly "hurt them."
Do we really need this? Would 2 caster classes be enough?