As for the PS5 and neXtBox (coining this now) they wont be fully backwards compatible. That is to say... software wise they might be very close to backwards compatible, but do you really imagine that neXtBox will have the port to connect the Kinect2.0 to in order to play Ryze or Fighter Within?
How about the PS4, do you think it will be completely compatible with PS4's PSVR? The move from the PS3 which is used with the PSVR, or even things like the controller from the PS4 when you need to use the motion control and the little mouse touchpad thing on it? I think games which rely on specific hardware... Skylanders and Starlink both come to mind again where they have a thing which needs to connect to the PS4 in order to play. I can't see both systems being able to give 100% backwards compatibility.
The few people who still want to play Kinect-2.0 games could buy this adapter for a USB-connection:
It works on Xbox One S + X and Windows-PCs... it shouldn't be hard to extend that compatibility to the Xbox Scarlet.
Sony already announced PSVR-compatibility for the PS5. Since the headset + camera get connected via USB and the move controllers are using standard bluetooth technology, it shouldn't be a big problem either.
With USB + bluetooth available, all the Scarlet + PS5 need for compatibility are proper device drivers. That shouldn't be impossible for big companies like Microsoft and Sony.