I'm going to tell you a very basic truth as it applies to me.
Leveling a twink out of bracket ultimately sucks more in regret than having another 85 does on being cool. One exception, unless your 70 actually WAS your BC main like my UA/demon lock. In which case I'm thrilled beyond words I'm 85 instead of crying about how my once proud Goddess of war was now 2 shot fodder.
My insanely geared glaived, red drake riding, "the Argent Champion" rogue was my old 49. As spoiled as that character is in gear, mounts, titles, companions etc. by any stretch of the imagination I wish..... I wish that there was still a 49 bracket for it to play in.
Listen, trends come and go on this game. We once had Naxx enchants that went bye bye, spell threads and leg armors >50 went bye bye, uncapped gathering professions went bye bye. 70 XP off BG's and arenas are not going away. They're frustrating to be sure right now there since a competitive random is rare and you seemingly have one side faceroll the other. You may complain about OP healers, OP burst etc. These are all arguments people had about 49's. Yet I miss 49's.
Unless you're pushed up against the server/realm cap on character slots my advice is... don't level out. The bracket has issues, however 85 PVP has even more issues. You don't know what's coming up in the current year with redesigned spell books, trees and itemization and how it will affect us. In the meantime... suck it up and stick it out.
You can never de-level your toon and you will always reminisce and wax fondly about how great your toon was at 70 when it had everything it could have and more. You take it to max level you'll never have that level of satisfaction with it again.