17 inch is a much smaller market segment. And while you are right that the 16 inch screens are indeed smaller than 17.3 inch 16:9, the difference is made smaller by the move to the 16:10 format (16:10 screens are bigger than 16:9 screens at the same diagonal). The 16 inch 16:10 screens offer exactly the same screen height as the 17.3 inch 16:9 panels, so you "only" lose some horizontal space (roughly 1.5 inches).
You could correctly say that this can still be problematic for some users, but the size difference is certainly much smaller than between a 16:9 15.6 inch and a 16:9 17.3 inch screen.
The main advantage of the P17 is not the "higher Total Graphics Power (TGP) of its Nvidia GPUs". The main advantage is the 17" screen.
And here is the P16 a complete failure. It might be a great successor to the P15 G2. But for all users looking for the screen space without an external screen the P16 is just not an option.
Let's hope that Lenovo gets sane again and offers a 17" workstation grade ThinkPad soon. Otherwise at least my next purchase will be from a different brand.
The availability of the new Lenovo workstation flagship, the ThinkPad P16, is getting closer. Now, Lenovo has published detailed specifications and manuals, which reveal additional information about the powerful work-laptop.