Essentially, the A7r/A7rII has created a race between Sony and Canon for a large group of customers, centred around former 5D2 and 1D3s users - those for whom IQ is paramount, but who would also like some functionality (e.g. AF) in the camera. Many migrated to the D800e/D810, but many more either have a large stable of Canon lenses, or certain Canon-specific lenses (usually tilt-shifts, or the MP-E 65, or supertelephotos) which they are unwilling to give up.
The race is between Canon developing a better sensor (in terms of low-ISO DR, which is the main reason many photographers in certain categories have migrated away from Canon) and Sony developing AF which can quickly and accurately focus using third-party glass. 'Third-party glass' being the key - without the ability to use Canon glass, the A7r would likely never have gotten off the ground in the first place, since Sony doesn't have the same sort of glass collection as Canon, and, even if it did, the photographers in play in this race are already largely wedded to Canon lenses already (the 5D2 having been a huge success attracting millions of photographers to Canon, or to digital for the first time). If Canon wins, they get the 5D2 users back. If Sony wins, they split open the crack that they've already produced in Canon's user base and take over this large segment of photographers (who will probably also buy more and more Sony glass as its collection develops and becomes competitive with Canon), leaving Canon with just the high-speed action crowd.
Nikon is the big loser in all of this - unless they can update their lens collection to be the equal to Canon's, including specialty lenses, they will lose market share whether Canon develops a new sensor and photographers migrate back (and stop moving to the D750/D810) or Sony wins and a Sony body with third-party glass (superior to Nikon's) becomes just as functional, in terms of AF and other areas, as the D810 and its successor bodies. Needless to say, Metabones is the big winner in the meantime. And Canon wins in terms of lenses regardless who wins the sensor/body battle, since Canon lenses are the generally-preferred lenses in each case.