Any nanotech origin for the Borg comes pre-Jossed. The Borg got their nanotech from Species 149. Assuming that there numbering scheme makes any sense, that was early in their carreer but still well after they started doing their Borg thing.
According to Memory Alpha, they didn't get their nano technology from Species 149, they got their procedure for reviving dead drones using nanoprobles from them.
I prefer the gradual transhumanist explanation, because it's so believable. Start with one small neurological implant that becomes ubiquitous due to it's utility, perhaps a wireless DNI that allows one to operate a computer and access the internet with thought alone. From there, you start adding features and designing new implants. Practically everything that the Borg has is of serious utility value to an individual. most woundly want all the implants, but many would want at least one or two. The DNI is a no-brainer, and super strength is nice, too.
Now, the problem comes in which the DNI implant starts being used for P2P chatting rather than just as a fancy keyboard and moniter. At first its no problem, because the brains remain seperate. But firmware updates that allow more inttimate mind sharing are inevitable, untill you end up mini-collective clusters,
Fascinating drama you have there.
Origin stories don't need drama, they need plausibility. How did humanity come into being? Evolution via natural selection over millions of years. Klingons? Same thing. Vulcans? Same thing.
Besides, it's very frightening and very poignant to understand how minor sociological and technological changes over time can end up transforming a race much like us into something that is totally alien in its mindset. If the Borg are victims of disaster, that takes something away from them. If they are the product of the same sociological forces that shape and mold us, forces and are, in and of themselves, good, then we can see ourselves in them. And more importantly, they serve as body horror on a societal level, a dire warning of what our grandchildren's grandchildren may become as our society progresses.
Really, Evil Being X transforming an entire race into drones is cliche and it's safe. Space Hitler is always a great villain, not just because one can hate him without feeling bad about it, but because he assures us that it really can't happen here.
But a series of decisions, a series of transformations, each of which were individually good and right and widely accepted, producing something so horrible, that at least makes you think, which Star Trek is supposed to do. Everyone knows that the road to hell is paved with good intentions, but sometimes it is paved by good actions, too. We cannot know how our decisions today will impact the society of a thousand years from now. And it's hard to come up with something far more frightening than that. It's not just that the Borg are genocidal conquerors from our point of view, but that we may very well become like them in a few thousand years time.