Actually in the film days there have been quite a number of SLRs with a fixed lens. I think the earliest SLRs have all been fixed lens designs.
Its just that the invention of the EVF such a solution seems to be weird nowadays.
I would always say a single lens is the optimal number of lenses for any camera, including any system camera. A system camera just opens up the possibilities which lens it would be; but it doesnt force you to choose any specific one.
For example I would totally go for a single Canon EF 85mm f1.2 and mount it to a Canon EOS 5D. It would be a specialized camera just for portraits, obviously.
Or if I would get a D750 or a second D600, I would lower my number of lenses per system camera from 4 to 2; I would probably mount the wide angle lens permanently on the D600 and the telezoom on the D750, unless its bad light, at which point I would switch to my two prime lenses.