Nah, it's on Holy Priest to be mediocre at everything.
Seriously though, it's probably true that Druid is
already inferior to both Shaman and Monk in terms of mobility, in the current patch. I just don't agree that a change to Displacer Beast really puts you that much more behind them. Or at least not worth a "sky is falling" reaction especially when things aren't final. However, I also think casting while moving is an unhealthy ability for the game for Shaman to have, and especially on such a short cooldown when talented since it allows them to cover pretty much every fight's movement requirements without any downside. It will be interesting to see if they touch it.
The strongest Resto Druid niche is survivability now, not mobility, I would say. And it will continue into BFA assuming no changes to Bear Form or its relative strength in PVE post-stat squish.