Well, there are several non-mass-momentum synergies here: Devourers, flying Discords, and Amber Nymphs vs Sanctuary, Fractal RoL/Hope vs. Unstable Gas, non-Water swarm vs. Flooding, . I fully approve of this as a viable option for the future Gravity shard.
Two reasons for keeping it a spell: First, it will likely be like sanctuary where the opponent needs to have a full turn without it before the protection goes away, even if destroyed before. Second, if it is a permanent, wouldn't it loop on and off?