As a non too frequent AC customer albeit one that flies only paid J/Z or award J when I do travel AC I don't find upgraded passengers to cheapen J class.* It's rare that I even know who has paid for their J seat and who has been upgraded.

*caveat: I do sometime feel cheated when I have paid for J/Z and I don't always get my meal choice knowing that an upgraded elite has perhaps "stolen" my selection. I also feel cheated when in a Y segment on a J award and upgrading elites take priority on getting a J seat that is otherwise unsold.