Just to clarify, I believe you mean Nature's Grace, not Nature's Swiftness(that's the "make next spell instant cast")
Anyway, multidotting might not be as punishing as it is atm, you're right that a bit of the min/max game play is gone with snapshotting, but the more common (and far more important) problem is that a lot of players struggle to keep a 100% dot uptime on multiple targets at the same time without refreshing it way too early. This has been made a lot easier in some ways (30% pandemic), but harder since you can't cast one of your dots half the time.
I still think we will see a huge gap between the best boomkins and the mediocre, maybe not as big as now, but definitely noticeable. The gap between the best and the decent boomkins might be smaller (which sucks, I must agree).