In the past, many other companies also made rangefinder cameras to compete with the Leica Ms; they even use the Leica M-mount! Minolta CLE, Konica Hexar and the Russian clones were all pretty popular and capable cameras, but didn't affect the sales of Leica Ms much. I believe people who buy Leica cameras are buying more than just a photographic tool; they might be buying Leica for prestige, as investments, and maybe even as a status symbol. This may change slightly in the digital age for camera bodies; I don't know of anyone who will pay full price of a used M8, or M9, when the newer cameras are more capable and in some cases, cost less. Lenses are different, and this brings us to the question: Why buy a Leica body when you can buy the NEX full frame and use Leica M lenses for a lot less?
No doubt, many will still buy a Leica instead of the A7/A7r, for various reasons, but for the majority of us who want a small full frame camera with a short lens registration, will no doubt buy the Sony. True, most of us will probably never buy a Leica regardless of Sony making the A7/A7r or not, but for those who can afford the Leica, but are sitting on the fence because of justification for the higher Leica price, the NEX A7/A7r maybe sway them to buy the Sony. So yes, for those who can afford a Leica, and actually use the camera to make pictures rather than using it as a status symbol, it would make no sense to buy a Leica, unless the build and handling is more important than image quality, when there is now a much cheaper alternative. How large is this group I have no idea, but if I were a Leica Exec, I would be worried.
Fall leaf - Sony NEX-6 & Leica-R 90mm f2 Summicron at f2. Click for larger.