I understand brightness can be a problem on DSLR with really big AF sensors (less light reflected upwards in the viewfinder), but that is marginal.
Here are relative sizes of some cameras I own (with a 50mm 1.4 lens on 35mm equivalent FOV compared to a 100% and 1.00x viewfinder):
1) OM-1n has 97% coverage and 0.92x magnification, effective size of 0.89x;
2) Nikon FM2(n) has 93% coverage and 0.86x magnification, effective size of 0.80x;
3) Mamiya ZE has 94% coverage and 0.85x magnification, effective size of 0.80x;
4) Nikon D200 has 95% coverage and 0.95x magnification, effective size of 0.60x (adjusted for crop);
5) Nikon D7000 has nearly 100% coverage and 0.95x magnification, effective size of 0.63x (adjusted for crop).
So clearly there is a lot of difference in between the crop and full frame cameras I own. Now, I tried both the D700 (from a friend) and the D810 rented two times. The D700 has 95% coverage and 0.72x magnification, for an effective size of 0.68x. The D810 isn't that far from it, with 100% coverage and 0.70x magnification, for a 0.70x effective size. They are slightly bigger than the viewfinders in my crop cameras, but not by such a huge margin as my film cameras.
So, to reiterate, is there any technical reason we can't have full frame DSLR viewfinders to be at least closer to film cameras?
As for the DK-21M, I tried it and it's a pain in the ass wearing prescription glasses. It does make things bigger, but it hides parts of the viewfinder in the process, so I have to move around to see every corner. Though that's the tradeoff with a crop camera, so I live with it.