I like the idea of some sort of horizon being established from the beginning, and I think the horizon could be related to any of: time passed, block height, market price.
Offhand I wouldn’t think the horizon would be an automatic trigger for the fee to disappear (i.e. a pre-programmed sunset), but rather a trigger for the fee to be revisited by the community. Revisitation might mean elimination, but could also mean adjustment up or down.
The idea here is to go from stability to stability in an orderly fashion, while allowing for changed circumstances to be addressed by a changed set of participants.
Between the beginning and the first revisitation, the issue would be considered as temporarily settled and allowed to work out it’s results as it will.