IMO, one of the reasons 25m is falling in WoW is that there is no greater reward for running 25m over 10m. You still get 1 piece of loot per 5 people in run and it is exactly the same loot in 10m as 25m. No real benefit to running 25m and it is easier to get 10 people together than 25. As pointed out in the posts before mine Rift eliminated this problem as the 10-mans are entirely different runs from the 20. They share nothing. So there is actually incentive to run them alongside the 10s.