According to DxO, the sensor of the D800 rivals the medium format cameras, but we don't take photos with a sensor. It's the whole package:
- Body with sensor, cpu/dsp, electronics, ergonomics…
- Lenses
- Accessories like flash, tripod, head…
- That a grand total of 13 Nikon lenses out of the 258 lenses that Nikon ever made
- Nikon is currently making 93 lenses (quick count through their catalog)
- Forget about the TC converters
I'm not here picking on Nikon. I think that Canon would have the same problem. The camera manufacturer that would be in the best position for a 30+ mpx sensor is Sony (which designed and manufactures the 36 mpx sensor for Nikon) with their Zeiss lenses.