In "Descent" there's a screen readout on the Transwarp Conduit, and it says that it exits into the Delta Quadrant. As in, that's where it goes and Starfleet knows that's where the Borg are located. It's how they knew the Borg were there when they said so in FC.
"And they did come back. BOBW could have easily been their last appearance."
Which wouldn't match up with what Guinan said about them in Q Who?
Sure, they could have done so and gotten away with it because it was TNG and not VOY, which is just double standard but they'd still be out there without the plot being resolved properly.
And...? What connection does that have to the Borg? Please explain the course of logic that lead you from "The Dominion run most of the Gamma Quadrant" to "The Borg run most of the Delta Quadrant". DS9 said A. TNG never said B.
The Dominion are an expansive aggressive group that managed to apparently take over most of their home Quadrant (DS9 keeps flip flopping on whether they really rule the GQ or not), the Borg are older and more powerful than them so it makes sense that they'd similarly rule the Delta Quadrant.
The alternative would be for there to be other species in the DQ that can fight off the Borg and rival them for control of the DQ, and as we saw in VOY the audience can't stand the mere thought of any other species as powerful as the Borg existing anywhere.