Certain parts of your character really shouldn’t be account-wide. Aside from collectible cosmetics, new characters are meant to be blank slates. They’re their own person, not just extensions of your main. So they have no gear, no reps or achieves, no inventory, nothing else really done. You get to decide how to build them separately.
Making reps account-wide deletes the need to ever do them again. A large part of progression and content in the world now has zero replayability so I think it’s a big loss. Like take Korthia for example. Say you finish it within a month on the first character, now for the rest of eternity you never need to go back there again.
It’d be convenient for the rewards, but it really just deals damage to world activity and to alts too, who now have a lot less things to ever do. The best solution I think is for Blizz to add more catch up mechanics to make it easier (I dearly wish they would revive the MoP double rep commendations) and to not make reps mandatory for end game progression, or at least as less as possible.