Leica is going larger than 35mm for a reason. That same reason could very well apply to Nikon.
I think that Leica has a reason that does not apply to Canon nor Nikon. Leica needs a completely new lens system, with AF, electronic lens-body coupling, and such. At that point, it can be worth making clean break with old lenses, choosing a new format and lens mount, as Olympus did with Four Thirds, and as Leica did when it invented 35mm film cameras. (Both went for smaller formats in those cases though.)
Canon and Nikon on the other hand have a huge installed base of good modern 35mm lenses, and many such lens designs in production, which give them far more incentive to build on that lens system rather than divert investment into a far, far smaller market sector.
It is worth noting that all major makers of AF 35mm film SLR lenses, Canon, Nikon, Pentax, and Konica-Minolta, have developed all of their DSLR's to have backward compatibility with those lenses, while Olympus and Leica (and now Panasonic and Samsung) have chosen more radical routes with less or no lens backward compatibility.
Aside: I wonder if the "R system successor" hinted at by Leica will be 35mm film format (24x36mm), "M8" format (18x27mm), 4/3" format, or something else.
Remember when Nikon, not so long ago, stated that DX is all you need and all we will produce?
At the beginning of the auto industry revolution someone -don't remember where I read it-- said that humans can't go faster than 50 m/h before they go mad or something like that.
Nikon never said that DX was all it would ever produce or all that anyone would ever need; it was far cagier, talking about "no current plans" for 35mm format while also saying that it was keeping an eye on the technology.
And why do people think that one alleged misjudgment by one usually anonymous person is much of a reason to disregard any prediction that they do not like? Try at least judging the evidence and arguments offered for the predictions!