The GP stated that "you can't even put full-frame lenses on crop-frame cameras" and appeared to be confusing the limitations of Canon's EF-S vs EF mount.
My comments were addressing that and I mentioned 1987 to try to be specific about what I was referring to. If you read that as a contradiction, I apologise for not being clearer, but suggest as well that you consider the context of my statements.
Canon's switch to the EF mount allowed them to move away from mechanical focus and concentrate on a completely electrical interface between body and lens. This gave them a significant advantage in the speed of their auto focus, which is why they captured the sports and (to a lesser extent) nature market in the early 90s and which gave them a significant lead in the early DSLR field.
Nikon AF lenses rely on the body having a focus motor. Most do, but the smaller, lighter Nikon bodies do not and therefore while the AF lenses work, but don't have auto focus. AF-S and AF-P2 have focus motors built in and will work on either bodies that have a focus motor, or newer, lighter bodies that do not. So, while all AF type lenses will fit the Nikon body, some combinations will not have Auto Focus (usually older lenses with newer, lighter bodies). More, when canon went from FD to EF, they sold an adapter to allow older glass to be used with the newer mount. It wasn't ideal, but nor was it 'cannot use'.
So no, Nikon isn't UNIX. They've hacked on their original mount, produced models that aren't completely backward compatible but are only 'mostly' so and were slowed by their need to continue to support their large customer base. Like Microsoft (but done 'right', perhaps). Canon made a clean break with their older tech to implement something new that gave them some significant advantages in the short term. More like Apple.
ref: http://www.nikon.com.au/en_AU/...
and
https://en.wikipedia.org/wiki/...
Caveat - I'm not and have never been a photographer, but I've had friends and partners who were and have sat in on enough discussions, conversations and comparisons to have an interest in the technologies and processes. The competition between these two companies has been nothing but beneficial for customers.