Why should I be prevented from getting Mastery 6/7 because I don't have essence?

Maybe if I could earn chests more than once a week I'd be okay with this but honestly, why do you need essence for Champion Mastery? Like, how does that make sense? This shouldn't feel like some kind of minigame where you're thinkin "Oo I wonder if I'll get the champion shard I need this time."

That's not exciting. That's inconvenient.

Let me earn a champion's mastery by playing that champion. I don't want to be forced to buy luck-based chests to show my mastery with my favorite champions.