The rewards system for the leaderboards will all be removed since the leaderboards will be just cosmetics. "Maybe" they would pour it all on the champion and make it like they share a big pot.
OR
it's also possible that they would still count the top AFTER the 5000 rating but use the 5000 rating as max for the rewards earned per battles, focus point, etc.
There will be leaderboard in champion league (and only there) anyway.
5000 rating is cap for reward calculation. But players will be able to reach higher ratings.