Source is this video:
Windows Was The Problem All Along - Dave2D
We could obviously compare performance between windows and steamOS before on the steam deck, or between windows and Bazzite on other handhelds. But this is the first time we have had official windows and SteamOS builds for the same hardware.
Realistically the design goals of a gaming OS vs a general desktop OS aren’t that different. You want to balance performance, batterlife/power consumption, and making sure it withstands insane abuse by users and software doing anything you could never imagine that nobody should have ever tried to do. About the only design goal that separates SteamOS from Windows is fleet manageability features
You are forgetting backwards compatibility with ancient software that Windows still supports after 30+ years.
A lot of businesses need that in order to function.