Champion mastery decay

I think champion mastery should eventually decay if you don't play the champion for a while to accurately represent your skill level on the various champions you play. I have maxed out mastery on quite a few champions, but I don't play some of them anymore, and on the odd games that I pick them up and take them for a run, people assume I know what I'm doing, when, in reality, I haven't played the champion in months and am really rusty, which sometimes leads to flaming. If champion mastery decayed, it would give an accurate representation of which champions you play, and how well (or often, at least) you play them. Thoughts?