I think Rebellion is still a good thing for SPl even with moderate packs buying.
Mobile game without a deep and narrative gameplay need to artificially maintain hype and user engagement with new stuff coming out regularly.
In narrative games, the story maintains players in a cliff hanger situation keeping them engaged until the end. In mobile and especially PvP games, there is nothing like that and the game can rapidly feel empty or boring.
The big problem of SPL imo is the lack of new features and variations. Also the PvP system with rating system that reset every 2 weeks completely annihilate the feeling of progression that keep players engaged.