Except that there IS a scenario where that is actually possible and that is Windows 10.
Look, I'm not saying there hasn't been a huge, complicated and expensive push to do that; I'm saying it was a waste of time to solve a problem nobody (outside Microsoft) had.
There aren't any third parties desperate to release games on the PC but their Xbox only expertise made it impossible to do without UWA.
Does UWA mean the PC gets some games it otherwise wouldn't have done?
Yes, but it's not because of UWA, it's because those games were cock-blocked by the Xbox division in the first place.
Does UWA mean the Xbox One gets any games that would otherwise not be headed there?
No, because the Xbox division won't let anyone release games on xbox via UWA, all developers must go through the ID@XBOX program
Is a multi-format developer ever going to pick UWA and roll their own engine instead of picking a multi-format engine in the first place?
Seems super unlikely, doesn't it?
Going with a multi-format engine gives you, you know, all of the benefits of an engine, plus the ability to export to multiple platforms including the console that has twice the userbase of an Xbox and the PC storefront that has sales orders of magnitudes higher than the W10 store.
If you don't want to believe any of this is true, fine, whatever, I don't care.
I'll just leave you with the unanswered questions.
If UWA is so beneficial to game developers in multi-platform development, why aren't any of them using it?
If UWA is so beneficial to game developers, why are the only comments from them outright hostile?
If UWA makes porting so easy, where are all the ports that weren't coming anyway?