You're saying every cross platform game is using one fifth of the console/PC's potential
No, I'm saying that even on totally different hardware you can get to exactly the same game if you are not perormance-oriented.
If you are so much about one platfrom for all devices, why phones are different? Let's make one API for everybody, including phones, we can even reuse UE4 or Unity for that.
But as porting is cheap because of hardware similarities, games are still targeting it.
No that assumption is incorrect. It's not like anybody "ports" anything, developers just write the code with all platforms in mind.
Last time we seen a real port (Batman, I'm looking at you) the performance was so horrible, it was even removed from distribution.
XB1 is an extra 30%
That assumption is also incorrect we will see more and more third party PS4 exclusives in the coming years. When the current games started it was not obvious that XB1 install base will suck this much.
I would say that if PS4 games were harder to support on DX11 we would see a lot more PS4 third party exclusives right now, but because it's easy, multiplatform developers may go DX11-only route, after all it's easy to make DX11 game, there are a lot of DX11-savvy programmers and it's easy to support console versions: just reduce resolution and framerate.
The net result of this approach would be much less tech excellence and much more "last-gen graphics, but now open-wolrd" games (see Witcher 3).
The world is full of dirty developers who actually use and rely on the CPU. Bastards!
It's because the platform-holders decision to support DX11-style programming was a mistake. Even PS4 supports it, at the cost of the performance.
And some of these ports were lousy because they didn't want to invest in refactoring the engine.
I don't think they were Batman-level lousy, i.e. you argument is invalid again.
The bottom line: at the PS2 time Sony did not give a shit about compatibility and ease of development, and still it was a golden-age of gaming. Right now PS4 is easy to develop for, but the golden-age is as far as it gets, I would say 2015 is a golden-age of PC-gaming, mainly because of these wrong decisions.