12-14 million/year would be fine.
The Wii Sports/Fit/Brain Training crowd doesn't want to pay actual money for games anymore, that's fine. Nintendo doesn't need them.
It's more about creating a stable base of users that you know are going to stick around and can hopefully buy actual games like ARMS or Xenoblade 2.
75 million is a good target for Switch, and Nintendo can also co-opt this by simply making Switch a larger ecosystem that isn't tied necessarily to one device. So keep coming out with new Switch models, not just cosmetic redesigns, but use new Tegra chips as they become available and cheaper, like tablet makers do.
Library scales up no problem, and you get more people buying more hardware as the consumer is given the choice to upgrade when they want. Because Switch basically collapses both Nintendo hardware lines into one I think Nintendo has the option of making more Switch models, yes more than what the 3DS has.
By doing this they could get 100 million because what is a "generation" would be blurred and you'd have quicker turn over or double dipping of hardware buyers.
Apple does this, they basically have two generations of hardware chips in the iPad family selling alongside each other at the same time ... the iPad Pro is basically a full generation past the chip that's in the iPad mini.
