Now that next gen is here, diminishing returns is a real problem

[Zappykins]

Yes, we are. It's not just more polygons, but different ways of processing that needs to change.

You have probably seen this before, but it shows well what is going on:

The last one is ten times more accurate!  That is a whole giant whollop of processing power to pull that off.  Just because next gen is @10 times more powerful, doesn't mean it's going to look 10 times better. 

Now we need changes to other things, like ray tracing and light scattering to really make things look more realistic (aka Avatar like graphics.)

Remember back in the PS2/Xbox days.  While the Xbox was like 2.5 more graphicly powerful than the PS2, they looked rather even on most games.

DirectX 12, and most likely the next big upgrade to OpenGL, are going to be a few more years away.  Then it will take developers a while to figure out how to use it.  Things are going to take a nice step this new gen.  But for more realism - which seems like what you might one, that will be at least another gen away.