Champion "mastery" is mastery only by name now.
So, if you want to increase mastery on a champion you have to almost exclusively play other champions in order to unlock chests. Let me repeat thT - if you are trying to increase a champion's mastery rank you need to play anyone BUT that champion... or shell out real money.
Yes, I understand that they wanted to give people who own all champions something to spend shards on - but why in the love of God did it have to be mastery system? What's the point of champion mastery if it has near zero connection to actually playing that champion? Mastery ranks 6 & 7 aren't even worth of being called that. Grinder rank 6-7 or Spender rank 6-7 would be more appropriate.
Mastery should be a sense of accomplishment, not 20% actually playing a champion and 80% grind/pay/luck of the draw. What's next? Moving up to any division above Silver 5 requiring X blue essence? Because that's basically same shit: tying something that should be earned through play to a defective crafting system.