You don't seem to worry about your PC games being dragged down by having to run on 5 year old PCs.
You shouldn't worry about your PC games being dragged down by the original xbox one.
Well, there is no mandate for all current PC games to run well on all past PCs... and well specked PCs from 2015 had 16GB of RAM, a geforce 970/80.
and this is a fact that most games released aren't AAA, so they often scale pretty well to very low specs machines to begin with (Even some big titles don't require much to run... Fortnite anyone).
The idea here is that at the very least big budget first party titles should lead the way in therms of showing off what the system can do, if they need it sacrifice compatibility with existing hardware.
That being said, there still can be some obvious advancements that can happen, anything that can live in a special config file and changed without changing the gameplay:
- resolution
- frame rate
- better LOD
- higher quality textures
- better lighting effects (add Ray Tracing effects when the games runs on the new hardware)
Now the sacrifices (or tolerate games running at 15fps to sell more new consoles)
- Limit I/O requests from the HDD, which means slow moving open world games for another couple of years (any game that streams assets really, which is most of them)
- Limit A.I. to what the Jaguar CPU can handle
- Limit the number of interactive objects in scenes to whatever existing hardware can handle
- Limit the number of players in multi-player games
For examples of this just see third party games from 2013 - 2015, and compare the level of animation, etc. compared to true PS4 games of the time.