Watching the end of the movie now.
You're right, all V'Ger did was turn off the Planetary defences, everything else was still turned on because Starfleet was still able to calculate and observe what was happening as the V'Ger's weapons were being positioned in orbit for a culling. I was reading a novel recently, about how massively terrified some admiral was, which in his mind justified him into doing awful things in the name of planetary security after V'Ger walkd through their firewalls like they were made of tissue.
Stupid Star Trek 4.
If they hadn't disabled that comet gun from Terra Prime afer it proved to be such a tactical liability, because only an idiot wouldn't have demolished it the following day, I wonder how it would have fared against V'Ger?
(The alternative to demolishing the comet gun was to build 40 more so that, taking over less than half during any future coup d'etat would be a losing hand tactically.)
However, V'Ger did destroy a Starbase in the beginning of the flick. That's gotta be a couple thousand for a body count in the least? not the millions I was hoping for, but still nothing to sneeze at.