Arena does not serve a purpose anymore. If anything arena should be changed to constant level 1 scrims using the crystal format. They are not the scheduled points giving crystals, but exposing of the format and playstyle to new players. This introduces them to crystals with at no cost and allows them to find players they may want to form a team with.
MMR only works when you have enough people to create a hierarchy. At the moment there are not enough consistently doing crystals to justify this. You can play crystals every day and run into the same team 3-4 times over the course of the week.
Even on 1k population servers.
Infamy does not serve this purpose however, just because it's principally farmable stat.