Great illustration. Similar diagrams have been around, but this one's cleaner than most. It really shows how complex the dSLR/SLR design is, compared to mirrorless (or, going back decades, rangefinder) designs.

The story of dSLR and mirrorless is more complicated than the word, 'versus'. I do think that mirrorless schemes will eventually replace reflex viewing schemes. However, at the current time, both systems have distinct advantages and disadvantages. As microprocessors and sensors and OLED displays and EVFs become more capable (quicker), there will be faster adoption of mirrorless gear.

Canon and Nikon getting into the mirrorless full frame fray will (longer term) be great for the industry. But don't rule out Fujifilm, Olympus, Panasonic, and Sony. Each has a unique feature set to offer.

It's basic human tendency to want to pick a "best" of everything. However, "best" is nearly always relative to the task at hand. It behooves each of us to do our homework to be reasonably sure we get the right system(s) for our needs. I gave up Canons and Nikons a few years ago for a specific feature set I needed — a balance of stills and video capabilities in a small package. It was the right choice for me. But I agonized over it for two years before I made the jump. It was worth the time to research my options!

Nikon at least used to make it's own optical glass and lenses. As far a I know Nikon uses as most companies do sensors from various vendors. The Nikon D810 uses an older version of the same Sony made sensor as the Pentax K-1. Canon makes it's own sensors though. Designing a camera is one thing, building sensors is another. Dell designs and builds computers, but uses intel or ADM microprocessors. Apple used to use Motorola CPUs, then switched to intel. I hear the are proposing to make their own microprocessors in the future.