I've read somewhere that Sun Valley was originally supposed to be just another feature update for W10, 21H2 in this case. But that plans at Microsoft changed due to marketing reasons. Is there any truth to this? Honestly it would be very confusing to see a "Windows 10 v21H2" which looks and acts so different from earlier versions... and was glad they decided to release this as a whole new product.
From what i have seen Sun Valley was created entirely outside of the insider program (for use on OEM systems with touch screens), initially, later they changed it to 11 and we got to test it and it got released as "follow up" for 10. And it still is win 10 with some new features and some hardware requirements. Windows 10 21H2 is just a minor update of the already running 19041, 19042, 19043 and now 19044.
Windows 11 also has the same 10-year lifecycle. This is one thing Windows-as-a-Service didn't change.