Getting old with the GP disparity - every level is having this problem besides Kyber 1

Being is a lower tier for easier matchs and better rewards is a mistake. You'll lose on rewards (except very specific circunstances which involve not going to a lower division, so it's not the problem you're talking about).
So even if players do this "for the rewards", CG can't adjust rewards to prevent that, because those players are wrong.

However, some players with big rosters are perfectly fine with a 50/50 win ratio at a lower tier, because it's less time consuming: it's an easy and swift victory when they decide to play 6 rounds out of 12 and they just don't engage the 6 other rounds. I agree that's it's a "fun" issue (not a reward issue though) because the opponent doesn't have a say in this situation. Whatever they do, the big guy decides who's gonna win.

Unfortunately, MM seems to work pretty well in the higher divisions, so it will be difficult to solve this problem without creating another one.