Janeway destroyed a major transport corridor while it was in use. So, in addition to crippling the Borg’s ability to traverse the galaxy (particularly to the Alpha Quadrant), an untold number of ships and drones and the Queen herself were lost in the implosion.
The Borg being the Borg are going to prioritize self-preservation, so it is going to take a long, LONG time to rebuild what was lost. For the next few generations, outside worlds (particularly in the Alpha Quadrant) are mostly safe from being attacked.
She didn’t completely defeat the Borg, just kicked the can way down the road.