And the 900p vs 720p in four years time is definitely a possibility as devs push the envelope. That's always been the case with every 3D machine, hacking off res over the years to use better per-pixel features.
That is the question really, lets say 4 years is a reasonable figure, would the next gen be out or about to come out by then? Depends how long this gen lasts, but then in that context you could argue that the XO has "kept up" as long as was needed.
If it takes 2 years for games to really start to push the envelope then it's possible that only at the very tail end of this gen that there are more than res differences, and by then people may be happy to play with it until the next gen.
Also just to note I agree with everything you said in your post. Was just adding my thoughts onto the end.
Lets not kid ourselves though; 40% is a reality - the question is, how many developers actually get to target it in a meaningful way besides a simple resolution gain? My guess: not many, especially considering using that 40% in a multi game project isn't free. It needs time and resources too.
Something I've said in the past is how much more can the 40% do, especially if rendering at a higher resolution.
40% may not be enough to actually do anything significant at all.
You have to remember that you have a lot more pixels to actually process.
Would people be happy with both versions of the game running at 900p but the ps4 having better effects? I have a feeling that's actually harder to sell than the bullet point of 1080p. (Personally I would rather the better effects)
Even both at 1080p, will 40% be enough to do more/a lot better effects across all those pixels? And then they may just drop the XO down to 900p to maintain pixel quality parity.
So yes the power difference is real, but it's not that simple.
All my comments obviously only apply to mutli-platform games.