At least in squad arena you can work on the current meta and have a team to climb ready in a relatively short interval.
In GA, if you happen to be stuck with a bunch of G8-10 toons from previous developments that were cut in half for one reason or another, you have a matchmaking disadvantage that can't be fixed quickly. It is something to work towards too, but many players would miss out on crystals just because they started the game when level 70 and G8 was the cap.
Switching the two reward systems is about the dumbest proposal I have read. Making GA rewards more appealing could have potential, so does diversifying arena.
Switching or merging the two would only cause more problems.