The Planet Killer was a one-shot destroyed in the end.
The Jellyfish were one-shots.
The Genesis Tech was revealed to be inherently flawed and thus wrapped up.
The Whale Probe was a one-shot.
DS9 did what you're SUPPOSED to do with a massive new threat: wrap them up in a way that ends the story appropriately even if you didn't destroy said enemy (The Dominion).
VOY had them in the Borg homeland, it wouldn't make sense for them NOT to repeatedly show up.
Now, if TNG had done more to wrap up the Borg instead of just leaving them alone (like finding the Transwarp Conduit that leads to the Alpha Quadrant and destroying it, or explaining that each Borg Cube is its own Collective and that One Cube didn't tell other Borg about the Alpha Quadrant, ANYTHING really) then that would be that.
Instead, they irresponsibly just left them out there waiting for their next invasion.