Hi,
I don't think Leica's competitive position is challenged by the Canon. Resolution-wise it is very close the 800 Nikons, but it is still selling well. So I don't think they sell them on resolution alone.
The CMOSIS sensor on Leica S (type 7) is a major step in DR and high ISO, AFAIK.
Personally, I feel that large pixels combined with ultrasharp lenses without OLP-filtering is not an ideal combination. More like a recipe for an aliasing machine. I am not shooting Leica-S, but I see a lot of aliasing on my Hasselblad V and P45+ combo, not only on textiles but even on water surfaces, so I feel the problem is real. Stopping down to f/16 works as an OLP-filter and the problem disappears, but so does a lot of apparent sharpness.
From what I have seen, 3.8 micron pixel pitch seems a pretty good compromise, on the S2 that would be 94 MP. That resolution would make the lenses some justice. That pixel size is totally feasible as all 24 MP APS-C cameras have 3.8 micron pixels.
So, in short, I don't think the Canon 5Ds or 5DR are dead ringers for the Leica S, but Leica's choice of pixel size doesn't make any sense to me.
Best regards
Erik