Champion mastery should be something you MAINTAIN
Skill can be lost when you stop practicing. This applies to any activity that requires a certain amount of mental and/or physical prowess. Physical sports, mathematics, painting, playing musical instruments, cooking and certainly League of Legends all fit this category.
Losing your lvl 7 mastery badge is not a nice thought, but how well you maintain your champion mastery could be a more accurate indicator of how good you are with the champion.
I have lvl 7 on
but I still feed on her ALL the time