Aside from team scorched (hey, there was a shader I really wanted a few seasons ago), I haven't touched Crucible much since before Witch Queen. I'm guessing most people don't have quite my aversion to PvP, but it's definitely possible to play Destiny in a completely rewarding way without ever having to play against another human.
Crucible would be missed by some if it left, but I bet there would still be enough of a core audience for Destiny that the game could live on mostly without disruption if PvP moved elsewhere.