Personally I would say that they don't milk their franchises more compared to other companies just that they milk the hell out of Mario. Also it doesn't help that they don't seem to be coming up with all that many new IPs hence giving the impression that they are milking existing franchises.
As an example it is a given that Nintendo will mostly release the same franchises every Gen while you have a developer in Naughty dog and others at Sony who have worked on a new IP every gen. By no means am I saying it's a bad thing that Nintendo will release a 2d mario, 3d mario, mario kart, SSBB, Zelda, metroid every gen as they are all proven sellers and Nintendo would be crazy not to develop them just that it would be nice if they took some more risks with new IPs.
By doing that they can appeal to a wider audience and they can discredit those who say Nintendo milks their franchises or characters.
Edit- As for Sony and Microsoft releasing more iterations this gen that is a given considering they are still developing for their current consoles while Nintendo supported the Wii for a significantly shorter period.