There's a general issue whenever you have two distinct methods of representing the same in-game reality, such as druids and nature clerics, or warlords and battlemasters, which is that the outcome of what they do within the world depends more on the meta-game choice of which method you choose to represent the character rather than anything inherent to the character itself. If druid is its own class, then nature clerics shouldn't exist, because they cover the same concept space.
I think I straight up don't agree with this. For a few reasons. 1) Nature clerics and Druids to me, aren't doing the same thing 2) I have no issues with two things doing the same thing differently. If two players want to build ninjas but one wants to go shadow monk and another wants to go Assassin rogue, I have no problem. I allows players to have similar concepts but vary in play so no one is doing the exact same thing. But YMMV, I guess.