I didn't want to post a one liner in the vr but it doesn't matter if the rise is big, if the Pokemon deserves to rise than it deserves to rise. If Zygarde had gained Thousand Arrows half-way through the generation and it was originally C-, would it be wrong for it to fly to A? My point is how low / high the mon is right now doesn't really matter if said mon truly deserves that rank.
I don't feel like posting in the VR just to respond to you but look at it this way: Starmie's main niche is that it is a water-type special wallbreaker with a really good speed tier. It simply has rapid spin as a collateral effect of using it. Greninja is both faster and has more coverage. This is why Starmie isn't getting a SMOU analysis and didn't rise to OU last gen until Gren was banned. Hope that explains it :)