First of all, there is nothing that cannot be done, given enough effort. However, I didn't try to talk just about the basic differences... Yes, you can do a system with a reasonable effort that shows different size icons and buttons for the UI for stylus and finger, but at least I'm not a fan of doing things halfway, I tried to talk about "optimal" UI's. Optimal UI's should not feel like compromises.

Information density is basically split in half for finger vs. stylus, meaning how many controls or UI elements you can put on the screen. If you have a plan how you could design a system-wide style that would automatically accommodate the UI's so that the amount of information and the amount, size and placement of the UI controls in any given view would work for both stylus and finger optimized formats, please let me know about it.

A finger optimal UI won't be stylus optimal UI and vice versa. At least platform-wise, since people are going through different applications, in and out, switching between them, doing different tasks, different views and windows, having some apps stylus-driven (hey, i need to take the stylus out) and others finger-driven (damn, i need to put it back in) will not create a satisfying user experience.

But also as noted in some of the comments here, nothing basically prevents "you" as the developers from doing otherwise