For long time players who have managed to acquire all the champions the new resource added through the Mastery Chests is just plain useless.
Nothing would be more infuriating than spending the time and energy to level up in mastery just to get a champion shard or champion essence when you literally have no use for it. It's just added frustration that dedicated players, don't need.
To fix this my suggestion is to either make the champion essence usable in other ways like buying more chests or keys, or even combine the two resources (champion essence and cosmetic essence) into one so that no matter what you get out of a chest, it would still be usable in the long run.
TLDR: Champion essence/shards are useless to people who own all the champs. Fix it rito plox.