The Destruction of the Unicomplex and accompanying regicide did not require Voyagers participation in the slightest.
But that wasn't part of Admiral Janeway's original plan. She had no interest in hurting or destroying the Borg, only in allowing the hero ship to safely slip into one of those transwarp conduits.
Arriving early and destroying the hub wouldn't really meet the Admiral's primary goal of getting Tuvok home in time for his pills. At least not assuredly so...