As a distraction from all the heat over the decay rate - I'm gonna throw a different worm into the blender.
How about relaxing the level limits on renown proccing on kills/chests?
PnP example: A small town is being harrased by a mob. They just want it dead.
John Doe the Newb shows up and kills the mob. = Yay! Renown increases.
Jane Doe the Elite shows up and kills the mob. = Meh, who cares?
Those people didn't care who killed their problem, just that it was handled.
Sure the renown for Jane the Elite's guild is miniscule compared to their overall presence, but her action was still noted by *somebody*.
I'm not advocating the level restrictions go away entirely, but maybe relaxed a little bit. That level 7 Vet character clearing up harbor quests for their favor should not go entirely beneath notice guild-wise.