Chest locking heroes

I like the idea of earning chests for great league play. I realize the limit of 4 per month to encourage spending for getting them faster. I just find it frustrating how a champion can only earn one chest EVER. Many players can only earn S-, S, or S+ with their main few champs. Once those are earned they can't get anymore. I think the 4 per month is good, but how about unlocking chests for champions each month? For example an Azir main can only earn a chest on Azir once a month, then it resets after 30 days.