I think the current rotation is good. For a new player, SGM offers an overwhelming amount of content to do, which sucks up too much time. It probably also puts off a lot of people, because they can't complete it. Once you've maxed out a few characters and can speed through things, things definitely seem more barren, I understand that. However, globally reducing the time is not the right approach imo.
If Rifts were remade to be more fun and less punishing, and versus mode offered more value, there'd be enough content for veteran players too