The a7rII is a bit better than the D810, the difference is of low significance in the real world.
At low ISO, yes.
But the A7r2 holds up much better than the D810 - and A7r - at high ISOs. Not that the latter two are hopeless, but the difference is quite noticeable at, say, ISO 6400-12800, shooting live theatre or music using eye detection AF. The A7r2 can match the 1Dx2 and D5 image quality-wise, while the 36MP sensor lags behind.
Now, Canon was indeed far behind, they have mostly caught up. Sensors are unlikely to make a big difference moving forward. This being true btw cameras but most importantly relative to the actual needs of most people.
All it takes is one leap in technology. The transition from off-chip to on-chip column-parallel A/D conversion was one such leap. CCD to CMOS was another.
BSI has the potential to deliver this - put a capacitor behind each photosite (possible due to the three-dimensional nature of the circuitry) and you're suddenly looking at possible ISOs several stops lower, and 20 stops of more of DR.
Or something replacing silicon and leading to a big improvement in quantum efficiency, leading to much less noise at the same ISO.
Or a multilayer sensor (like an improved Foveon) capable of distinguishing colour in each pixel, leading to improved colour accuracy and resolution and reduced artifacts.
Just see the success of the Fuji X-T2 with its Sony APS-C sensor. Do you think that Fuji is utteely concerned whether they will get access to the latest breed of Sony APS-C sensor for the X-T3?
I doubt Sony considers any APS-C sensors to be part of a line worth protecting and monopolising.