The Borg are capable of time travel--we know this from First Contact. Ok, we can say they don't travel back in time in the Delta Quadrant and THEN assimilate earth because they'd rather have 24th Century earth and not 21st century earth, check. But once plan A failed, they DID use time travel and took 21st Century earth. SO, once THAT failed, why wouldn't Borg still in the Delta Quadrant say "f it, these humans/Federation are way too much trouble. We'll travel back in time here, in the Delta Q, and THEN assimilate earth." I mean, ok, the Borg back in time were stopped from getting their beacon to work, thus stopping those Borg from communicating with 21st Century Borg, but am I really to believe that the 24th Century Borg in the DQ never found out any of the details of what happened in First Contact? They had to have known about 1) the destruction of the cube that invaded thee AQ, and 2) the time travel bit, because of the signal that connects them all.
At some point, why would't the Borg just use this devastating weapon (time travel) in complete safety, and change history without that pestilential Enterprise D around? Just saying.