Even after kess flung them 10,000 light years past borg space Voyger still encountred borg cubes and stations regualy and the uni complex was in easy reach in series 5-7. I have two explinations: 1) is that borg space was not a uniform shape but rather spread out like a spider with tendrals going off in diffrent directions. Kess just flung them past on tenderal of it but voyger still had to pass close by other parts snakeing out around the delta quadrant. 2)Borg space is not all connected. Infact to the borg borders would be irrelvent anyway. Instead it is in pockets like mold. The borg see a place they like and assimilate it and that then becomes borg space which explains why they are happy to go out there way to assimilate the federation, it would just become another pocket of borg space. Also what about the Krenmin. They where next to borg space and had tecnology better than the federation and possed a threat to the borg. Surely they should have been assimilated by now?