I actually sense some more internal rivalry in the group now for the center position than in the platinum era.
I think the center position matters more now than it did when there was Ai and Reina. Them being in the center seemed like the natural scheme of things.
But now, I feel something different in the group when Riho was a bit off-center in the most recent single. I think the other members have become more confident and really want to challenge Riho for that spot.
Management is clearly stirring things up in expanding the center stage to accommodate a few more people than what we're used to (1~2)
It could be that because Sayu has left that there isn't that element of that person who can stand out from anywhere. Sayu didn't need to be in the center because regardless of the fact that she isn't the most skilled among them in singing or dancing and she made her own space and stood out.
Now everyone is pretty much in the same place, just a little more skilled in some aspect compared to the others but all the factors kind of even out. I think rivalry is possible only in situations like this. I'd say rivalry was nearly impossible when Ai Takahashi was around because there's no competing with her in terms of singing and dancing skills and during Platinum Era, that was all MM could show non-hardcore fans.