The OP is talking about Apple putting tvOS on Smart TVs.
Perhaps I've misinterpreted OP, if so, please help me understand. But it appears to me that's exactly what Apple and LG have done on recent LG TVs.
I don't blame you for being confused, as I blame Apple for their recent horrible marketing and specifically their naming convention for their products and services. The Apple TV, Apple TV app, and Apple TV+ is just horrible naming.
The Apple TV app, just that, an app.
tvOS is an operating system that is on the Apple TV hardware.
Apple TV+ is a subscription streaming service.
The Apple TV app is the not same thing as the tvOS on the Apple TV.
Having AirPlay and the Apple TV app on your LG is not the same thing has connecting an Apple TV 4K to your LG.
This would be like saying that downloading the Apple Music app on Android is the same exact experience as using iOS, and it would be like turning your Android phone into an iPhone.
The OP is suggesting Apple should license the operating system (tvOS) for the Apple TV hardware on non-Apple hardware, like TVs.
I personally do not think it will work.