As for - 'A singular destiny' - it established the only surviving borg tech was fully inactive and all attempts to activate it will turn it into ash.
turned it to ash. Not "will."
A Singular Destiny
leaves the door open to the possibility of Borg-less Borg technology eventually becoming functional. It doesn't establish that it will, but the door is open.
If someone actually figures out how to reactivate borg tech - easy coming back card for the borg.
There are a thousand ways the authors of the novels could bring the Borg back if they wanted; no one is contesting that. It would just be a bad idea.