They're going to HAVE to drop support for the older systems. I don't really understand Microsoft's position here, a new console should mean exclusive games that harness the muscle of that console. Otherwise, what's the POINT of the new generation? There's a case to be made for the Xbox One X, but original Xbones shouldn't be considered for backwards compatibility. Microsoft keeps making decisions for their consoles that seem like they weren't thought through for more than a minute.
Sony, yet again, is doing it "right". Some games will run on both, with the PS5 having better load times/framerate/texturing, and some will require the new hardware just to run properly at all.
Honestly, none of this matters to me personally, as I'll be playing any Xbox games on my pc (they did manage to make THAT happen quite well). My Xbone didn't even get unpacked when we moved a year and a half ago. The PS4 gets occasional usage, and always for exclusive games. Sony's exclusives are spectacular, and well worth the cost of a PS4 (Pro, in my case) to experience. Microsoft has had a much weaker exclusives list as of late.