"Wouldn't it be way better to just lower the rewards in general and give reward bonuses to players who do have higher collection values based on their league? This would be a positive incentive instead of a negative one which is imo way better and allow all players to reach any league they are able to regardless their collection"
This is the best point so far!
That's how I also see it and I have yet to see a counter-argument. This would basically hurt a bit at the start (just like switching all card rewards to the system where DEC/Potions were included). Everyone would understand that it needs to be done though and afterward nobody would really have a negative feeling about it.
Now everyone is just seeing how they are blocked by a ridiculous paywall.