1. One would assume that DIR would want to continue to swell its ranks.
2. It's way more likely people will give up BM to go SM, but probably not too many going the other way.
Ergo - better to assimilate the sidemount than to watch sidemount leach your population away.
Lastly, I think that as more DIR people tout the benefits of sidemount, the DIR folks will themselves demand sidemount be standardized and brought under their system.
I see...