"Are the Borg actually Star Trek’s version of Mass Effect’s Reapers?" - A possible theory on the Borg’s motivations towards humanity and the Federation.
I've always wondered why the Borg would only send one cube at most on numerous occasions against the Federation, when we know full well their full strength in the Delta Quadrant. In order to explain this, I’d like to bring up a theory that explains the motivations of the Borg seen from TNG through the end of VOY. It’s certainly not a new idea since it’s been kicking around in various forms across the Internet for quite some time now. It’s basically the idea of technological “farming”.
The premise is that the Collective “farms” the galaxy’s most creative civilizations for knowledge and technological advancement. The Federation is clearly the ideal candidate for such a strategy; a free and open democratic society with incredible diversity that also fosters a culture of personal growth and intellectual accomplishments would be the perfect entity to foster great achievements in science and technology. The Collective was able to identify these traits in the Federation as highly desirable for their own ends. It most likely disregarded the Klingons, the Romulans, and the Cardassians as being too singularly obsessed with warfare, conquest, and racial chauvinism in order to provide the type of polyglot society that produces rapid innovation like the Federation does.
Once a suitable society like the Federation found, it must be stimulated to maximize the efficiency of its technological development. To stimulate it, you must make this society believe that they truly face an existential threat, that they are literally on the verge of annihilation if they cannot innovate or invent some major breakthrough to save themselves, but you must leave them some hope, otherwise they may just give into hopelessness and give up altogether. To do this, you attack them routinely, come very close to obliterating them, but fail every time as we see numerous times in TNG and VOY.
It may explain why the Borg has seemed to only prefer to send a solitary cube against the Federation. It may explain why the Borg, although possessing transwarp hubs in the Delta Quadrant, never launch a swift full-scale invasion of Earth. Even the Borg's temporal incursion seen in "First Contact" may not have been what it initially appeared to be. Instead of a full-scale effort to eliminate the Federation from the beginning, it may have just been at attempt to delay its progress so that it doesn't become too GREAT of a threat, but still potent enough of a society to still produce useful technological innovation for the Borg.
If the Borg wanted to destroy or assimilate Earth in the past, it could have just transported Zephram Cochrane into outer space or self-destructed the sphere over the Montana complex. The somewhat superficial damage created by the sphere shooting at the Montana encampment where The Phoenix resided suggests that the Borg wanted to only delay first contact with the Vulcans, thereby preventing the Federation of the future to form, but still allowing it to ultimately exist, albeit in a weaker but potentially more malleable state.
In my own head canon, this ultimately changes with Janeway's actions in "Endgame". By destroying the transwarp hub, Voyager inadvertently changed the entire calculus of the Collective. This may have been the final "straw" that broke the camel's back. After this incident, the Federation may have finally been viewed as an existential threat to the Borg themselves. The Collective could no longer afford the luxury of slowly stimulating and "farming" the Federation for technology. It was time to quickly deal with this threat once and for all, when it could still be done and before the Federation fully integrated the advanced technologies that Voyager brought back with it from the Delta Quadrant.
Cue
David Mack's "Destiny" trilogy and the REAL endgame for the Borg.
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Obviously, this is just my own personal head canon and it's definitely not conclusive or authoritative by any means. But I think it provides a useful framework to explain why the Borg, despite possessing the ability and motivation to completely wipe out the Federation, don't do that on-screen (aside from the meta-explanation that in doing so, "it would end the franchise").
