Why the Xbox Series X and not Xbox Series PC? One of the major reasons that PC gamers constantly upgrade their hardware is to help increase their unit's performance. Of course, there may be many other reasons as well which include new feature support or maybe to reduce the power consumption (and noise) in order to name a few but at the end of the day, most upgrades are purely for a boost in their unit's performance which includes enabling a higher framerate and/or upgrading the resolution.
This particular advantage for the PC architecture together with the compatibility that comes with the massive library of previous games that sometimes date up to decades is an essential part of what makes the PC so powerful and even beloved at the same time.
In a certain blog post about the Xbox Series X by the Director of Program Management for the upcoming Xbox Series X Jason Ronald, it was revealed that a collection of new details with regards to backward compatibility along with performance enhancements are en route with the upcoming Microsoft console.
If you were able to read the post, you'll then see that virtually every enhancement and value-adds actually come with the Xbox Series X and act as a courtesy of the AMD-built, PC-styled technology that is being used in the upcoming console.
To Microsoft's own credit, making backward compatibility possible with the Xbox 360 games all the way to the Xbox One games that released about five years ago was definitely no fall feat. Although the compatibility of the Xbox 360 games was previously universal, Microsoft still did an awesome job in ensuring that customers would not abandon their whole existing game libraries should they ever upgrade to an Xbox One.
Both the Xbox 360 and the Xbox One were leveraged completely with different CPU and GPU architectures. Since that very point, the iteration of the Xbox has also leveraged AMD x86 CPU along with an AMD Radeon GPU technology, which is both very similar to that of a PC.
Upgrading a PC
This is quite the same way that you can upgrade a PC from the practical Ryzen 3 APU along with integrated Vega graphics all the way to a much more powerful Ryzen 9 that comes with a discrete Radeon Navi GPU that wouldn't require any type of special additions in order to make it compatible with the previous existing game library and increased performance. This is exactly how the Xbox Series X's new hardware is built.
The blog post also explains the benefits of a much faster NVMe-based storage which is applicable even when running those much older games on the Xbox Series X, in addition to turning on the HDR where it was still not previously possible or even doubling the framerates for certain titles in order to enable much smoother animations.