On the system requirements angle, PC gamers generally don't care anymore, however, MS can certify a few standard tiers, say, 'xbox 2026', 'xbox 2026 premium', 'xbox 2026 ultra' and the software and hardware ecosystem follows those.
Microsoft can curate a store of games regardless of the nature of the hardware. The app stores choosing to let developers run wild has nothing to do with in-house hardware.
For the software modding and third party applications, they can have their software platform do this. Windows being open is a choice (and in fact one they wanted to roll back with 'S', but to no success. They could release/license out 'xBox OS' for this experience.
On the game discs, the consoles are largely killing this anyway, famously Switch 2 game cartridges are likely to be nothing more than a dongle and Sony selling variants with no optical media at all. Online entitlement is their direction, and the industry seems content to screw over second hand market (that's a bonus for them) and anyone with zero internet.
A GPU only costs as much as a console if you go for a GPU more potent than what goes into a game console. An xBox Series X equivalent GPU is like $250.
Most games that release for xBox release on Steam for PC as well. It's generally considered an easy port for bigger market reach. That's why pairing a game controller with a PC is so popular, and steam big picture mode.
I would not at all be surprised for the near future for 'xBox' to be Asus, Dell, Lenovo, etc devices under a licensing deal with perhaps restrictions around particular TPM endorsement keys and certified specification levels for performance and portability use cases.