One would think that Sr would be a much more fair measurement of success. It seems MMR takes one’s rank mobility into account. A purely SR based matchmaker would just find people of roughly the same SR and randomly select from that pool. Then survival of the fittest would do its thing, your sr would bump a bit, and now you are up against slightly better players the next round.
MMR seems by its very nature to measure how specifically you are doing that round compared to others for matchmaking purposes by being more momentum oriented than the base SR.
Why care about momentum at all?
IMO a raw SR matchmaker is what we need.