Why can't you set some ground rules requiring that in order to do Indoor line, they have to audition for Region, or do a solo, or a small ensemble?
Why is the answer "take away what the kids want to do" when there are ways to get them to still be successful AND become well-rounded?
Hell, you could even mandate that a student cannot perform on the same instrument for indoor as they do for marching band. Make those snare drummers learn mallets. Let your front ensemble kids learn to play bass splits.
It's possible, without saying "Get rid of it."