Well Leica's future will not depend on the S2 sensor size either and they even talk of future sensors (S3) that will replace it in the future . Building lenses for the current sensor size is not what leica will be doing, i would bet pretty heavily the image circle is much bigger than the current S2 offering, even the existing R lenses can handle even more than FF 35mm. On cannibalizing there own S2 offering is a maybe folks will buy it for the speed but many Mamiya shooters and there are a lot will not go to the S2 and switch systems, so yes i see a big after market for there glass. Look at Zeiss they are doing fairly well in supplying Nikon, Leica, Canon, Sony and others and even Voigtlander is making a nice profit from the Leica M8 shooters. I do see your point Graham but I also see a big after market too. I think what you will really see though is leica helping as a 3rd party vendor other companies develop lenses for there bodies, now that is a profit maker.
But I do agree this alliance is far from being settled and until something is really concrete on it, it is a guessing game. Even at the executive level that I speak to leica does not know either.