So this is mainly a question, if the game already works this way or not. I could not observe so I just ask.
We know that skills degrades the higher they are and at some level they won't degrade further.
What I expect is that the "no more degradation over time" value is tied to traits of the colonist so a trait that gives a +6 melee would set the target degradation of melee to 9 (base 3 + trait 6) while a trait that gives -2 would have as target degradation to 1. This pawn, no matter how many fist fights he goes through, will.. forget faster how to punch.
So, as said, my question is if the game already works this way, and if not if you think this should be the way to go
It's currently much simpler than what you describe; skills that are higher than 10 will degrade (faster the higher the skill) and 10 or lower will never degrade.
http://rimworldwiki.com/wiki/Skills
I like the idea of having traits modify how that works though.