If the Borg wanted to get to earth via transwarp, with a single cube or a fleet of cubes or whatever, I don’t see that there was ever anything stopping them from doing so. You both make it sound like Voyager making the request to help them get home would have given the Borg some kind of Epiphany regarding the situation. Nothing about escorting Voyager home would have put the Borg in a more advantageous position in terms of potentially assimilating earth than they were already in.
True enough, I was just amusing myself with the scenario.
I always assumed the only reason Earth didn't get assimilated was because the Borg were either distracted by other things or simply didn't want to go through with it for some reason (yet).