Are you saying next gen needs to be backwards compatible for you to buy a console day one? Because I'm pretty (very) sure both the PS5 and ...whatever the next XBOX is called, will both have full backwards comparability with this gen.
This thread title is really, really hard to get my head around, already poked to see about having it reworded slightly.
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.
Fancy hearing me on an amateur podcast with friends gushing over one of my favourite games? https://youtu.be/1I7JfMMxhf8