I agree that 60MP is massive overkill for most people at the present time. As stated earlier, lenses will be the weak link, even if your computer can eat the files. Mind you, there will still be amateurs who will pay $5K for the lenses needed to tackle such sensors (just as there are buyers for the Leica 50 Summicron APO).
IMO the next evolution needs to be in affordable lenses that can tackle the sensors we already have. Forget 36MP, there are still a lot of Nikon and Canon pro lenses that struggle with 22-24MP at the apertures the lenses are marketed on. Any 60 MP DSLR will IMO be two lens generations ahead of what is well supported.
Everyone was hoping that Leica M lenses would perform flawlessly on the A7R, but this is not the case, not even close. Had it been, 36 MP would have look VERY different on the A7R than the D800E. Letting a Leica 24 3.8 Elmar loose at full potential would have made this abundantly clear, but it is hobbled in the corners. It still seems that Leica has the wide angle lenses, but not the platform. Others have the platform but not the lenses.
The FE lenses and A7R may end up 'great' when more lenses are rolled out, but something tells me its not quite going to give us a 36 MP 'Leica system' performance. But as the OP says, we have staggering flexibility.
All said, however, I do still think the A7/R is going to be a game changer with no small part being due to what the OP said. This is a camera that an entrenched user of another system can buy into (like me - EOS and Leica M). Maybe not perfectly, but well. It seems clear now that in order to get great results, many people will need to fill in some weak areas with native lenses (like the very wide lenses). This means, IMO, that Sony need to produce a couple of spectacular primes in the 16-28mm range. Forget speed, go for eye-watering performance right into the corners. My leica wides are not going to cut it. Its good that I bought in not based on amazing resolution (tho that is nice), but based on portability and dynamic range.
For those who are not obsessive about resolution being perfect everywhere (which should be most photographers, if they have any creative ability at all), the Alphas are a game changer. 50 ZM planar, 24 TS-E, 75 Summarit, 35 Sonnar FE... all in one bag.