I really enjoy my Leica II and IIIc. I use them regularly. But IMHO the Contax II and IIa were easily as good as the Leica. And the Contax IIIa is certainly one of my all-time favorite rangefinders. Except for the misfortunes of war the results between these two companies could certainly have been different. Though I do not mind it, I do believe that Zeiss Ikon would eventually have had to resolve the dual lens mount issue but that could easily have been done.
But, I still think that the Japanese would have prevailed in the end.