I think this is a horrible Idea. It can lead to a lower league being more viable than a higher league. Look at modern, bronze right now: With the numbers you propose, bronze players would get more rewards (10%/50000) than silver players (15%/110000). What a punch in the face if you just spent hundreds of $ to get your silver summoners etc. just to find out that it's overcrowded and you actually get less now... (i know chests are better...but still what a fail that would be)
It can't be that just because more people come in and spend more money to get to silver or gold, all the players already there will get diminishing returns because their league is becoming overpopulated.
Please look at competitive games with working mmr/elo ranked systems, where does the majority of players (the casuals) end up? silver/gold. And yes it's a bit different here (towards pyramid) because of the paywalls between leagues, but even your numbers here suggest this is how it's gonna be.
This is gonna lead to "either bronze or diamond+" situation where you kill incentive to rank up to the populated middle class.
If you think diamond and champion need better rewards, then start by increasing rewards for champion wins by 15% and diamond by 10% or whatever, but don't experiment with the sledgehammer like this. This a step backwards towards a ridgid and primitive system that would need constant adjustment depending on the amount of players in each league etc. And changing league depending on where the rewards will be best depending on the reward pool and population is only feasible for players with (a) loads of money to spare or (b) automation (splitting accounts, rental bots/assist...)
Also: How many of the people voting actually read this and understand the severity of these changes? because the title of the proposal ingame does not reflect this at all.