Except if a game runs on XB1X, it can be made to run well enough on XB1 (and PS4 and PS4Pro). The launch games for a next-gen, at least in the first year (past the really early up-ports etc), are impossible on the old gen without crazy amounts of work and a compromised experience.
Gah, I knew I shouldn't have opened this thread when I don't have time. So many posts I want to respond to, but I just want to touch on this.
Yes, this is absolutely the case if the older console is 7-8 years old (traditional generation). But if the older console is only 3-4 years old (rolling generations)?
As I've noted there were exclusives released for the XBO that do not target any other hardware than XBO that run perfectly fine on hardware 3-4 years older featuring hardware architectures that are significantly different. Go back 7-8 years and that becomes significantly more difficult. Of course, with the pace of technology slowing (GPUs used to iterate every 6-9 months, now they iterate every 1-1.5 years and sometimes longer) that becomes less and less difficult as we go.
This is even more true now as everyone is using GPUs based on desktop PC GPUs. GPUs which have to at some level (driver or hardware) maintain compatibility with older hardware. Sure graphics features may not be feasible on older hardware, but it's easy enough to enable those on newer games that target all the capabilities of the new hardware while not implementing them on the older hardware.
For example, hardware tessellation that is used in some current gen game is just disabled on hardware that does not support hardware tessellation or can be disabled for hardware that suffers too much of a performance hit from it.
Unless the rendering paradigm changes, all this holds true for the forseeable future. Modern consoles aren't like traditional consoles that ditched all hardware compatibility. Hell, even PS4 shares some graphics compatibility with the PS3 due to the PC roots of their GPUs. Granted, much of PS3's rendering had to be done on the PS3's SPUs as the GPU used was anemic compared to the competition, so in that sense it straddles the line between modern console and traditional console.
Going forward, it isn't like any future console can completely ditch compatibility with a prior console (like when going from PS1 to PS2). Even going with a mobile smartphone GPU sees a lot of common functionality.
Also, SORRY DSOUP! Your post I also really wanted to reply to, but I just don't have the time.
I did want to mention that yes, scaling down can be more difficult or even impossible if there isn't already a framework in place to facilitate scaling down. However, PC GPUs and PC infrastructure are designed to help facilitate scaling down at hardware, driver, and software (API and game) layer. Yes, only one of the current gen consoles is close to PC infracture, but all of them use PC GPUs as well as PC based CPUs.
That's why games that targeted this generation of hardware had virtually no problems running on hardware of a completely different generation when those games were allowed to run on PC.
Regards,
SB