I don't see decay working the way they think it will. The current amount of decay seems.... high. I think it will be scaled way down before long. I do like the idea of milestone levels. There should be a max to your decay drop.
A milestone every 5 or 10 levels would make sense.
The system also needs inputs for actual Achievment. Right now it's mostly about grindage.
How about Special Bonus Renown for Achievements like beating a quest / raid on elite and epic? How about making this renown immune to decay? If I understand correctly you can decay back to 25. Special Bonus Renown would increase this base. This could also be earned through events.