In my opinion , given what the Switch is it will sell between 70 - 90 Million , rather easily.
The 3DS sold 60Million and still has a year or two of life in it - great sales but shrinking sales when compared to the Original DS. The Switch is something very unique , Nintendo's correct assessment of the future , in my opinion, given that the smartphones are eating at the portable market and that their home-console has to compete with PS4 pro and Xbox One s and PCs. The choice of opting for an eco-system of consoles for the future is the correct one, similar to Apple with iPad and iPhone.
The Switch is the home console version of their eco-system of portables vision , Nintendo will in my opinion within 2-3 years release a portable Switch (3DS successor) - will have the same eco-system of games and most features of the Switch Home Console, maybe we will see a Screen with slide-out controls (similar to the PSPGO) - would enable a smaller form factor.
Dont be surprised to see 2 or more versions of the Switch - all with the same game library but with different form factors.
People should not worry about game support for the Switch - look at the 3DS , it has a very solid game library. Given that the Switch is MANY MANY MANY times more powerful compared to the 3DS it will see even more support from third party developers.
Nintendo and their partners need not develop games for two platforms anymore, they can focus it all on ONE SINGLE eco-system , the Nintendo Switch. The Reason the 3DS outsold the PSVita by miles are the GAMES, all those games are heading to the Switch - Pokemon , Mario , Zelda , countless RPGs etc etc.
So dont worry people , the future of the Nintendo Switch is almost already written in the stars.
In conclusion: I expect the Nintendo Switch family of systems to sell from 70 - 90 (maybe even break that 100mil mark) before we see a Switch Successor with even better specs (which most likely will still have a similar form factor to the Switch , since the Switch is the embodiment of all the Nintendo ideas of what a console aught to be).
The Nintendo Switch is the Closest product deserving of the name , Nintendo Revolution.
(ONLY Nintendo products I have ever owned are: NES , DS , 3DS)