The Leica R4,5,6 and 7 were all based on the Minolta XD7, they are good cameras, not deadly special and surely not in the league of the real pro cameras of the 80s, the Nikon F3, the Pentax LX and the Canon F-1N.
Exactly, particularly the holy, glorious, all-conquering, boundless, expressive, ever-expansive Canon New F-1.
Now, is it totally true that those Leicas are Minoltas in disguise, more correctly they are Minoltas that have different electronics, improved viewfinders and prisms, Leica assembly and QA, and an R-system mount. And sometimes i wonder,
what's wrong with that? Those Minoltas which were used as "base platform" are really really good machines: XE-5, XD7. I want to get myself an XE-5, it is very nice. So let's take an XE-5, do the Leicolta modifications and it becomes an excellent machine.
The problem, IMO, is the price of the lenses. And of course the lens choice is limited compared to Canikon. As a 1982 Leica advert said: "When you demand the very best, your choice is somewhat limited."
As for the Contax RTS system, the Contax/Yashica system as a whole, I think it was a great idea, and its "sister" (cheaper) professional camera, the Yashica FR-1, is a nice camera as well. Yashica's lens factory (Tomioka) already had a good reputation.
Why did pros did not prefer the Contax RTS system in the 70s? I guess it's either to:
a) already having bought Nikon lenses and accesories
b) lens and accesories choice not as massive and/or comprehensive as in the Nikon (or Canon) system.