Ironically, plenty of folks thought the jump from PS3 to PS4 was also not every big either. There will always be those who don't see much difference and plenty who do (no different this time, either). Diminishing returns is more about what goes in and comes out than what is perceived.
As of right now, no, we haven't even gotten close to the gains being diminished. Yes, it is happening (though not a fast rate) and we have a ways to go before we cannot put in the work to make those jumps apparent.
Plenty of people already feeling the leap from PS3 to PS4 was too small is kind of my point, and this looks even smaller to the naked eye this time around. I am speaking strictly from a perspective standpoint afterall. I have already said several times I am not speaking technically nor am I including the other innovations that will make this new gen more relevant.
As the title says visual analysis, I only see a need to give my opinion based on visual perspective from what I saw in the 4k streams.