i wouldnt be too worried.
a) when it hits people wont be lvling via pvp...they'll be doing all the new quests, exploring new zones, and mostly new dungeons. i predict long que times
b) the gear reset is not that huge. if you have 264 or 277 gear you wont be replacing it till 82/83, especially if you are getting an amazing 2/4pc set bonus. 251s will be getting upgraded in the first few instances though.
then even after people settle in and have alts and try out lvling via pvp again, they'll still be gone quickly and prolly bads like usual. so i'd say 70s will be fine. just get bis, and stick together in a group. i bet it'll be harder to be a solo bg hero, but just play a team game and you should be ok