They mentioned very high level on the worm hole science being basically novice level. They only had the one spot to jump from and they could only jump back and forth to those two spaces in time, but time was still moving equally for both locations, so a week in the future was a week in the past etc. By the time they got the tech down, and could jump they were down to less than a million humans left on earth.
So if they 'sent everyone back in time' you'd have a bazillion paradox's and shyt. Its why the nly people who 'came back' to recruit were so young, aka they weren't born yet.