Gameplay wise I agree. it's been a dud.
But graphically ive seen improvements that have made me sit there in awe. I understand that its mostly been incremental but there are times where all the new next enhancements come together to deliver truly CG quality visuals I never thought possible until we got to 40 tflops. But yeah, diminishing returns was a thing even with the PS4 generation in comparison to the massive leap we had going from PS2 to PS3. The asset quality and lighting upgrades were basically the main things we saw get improved going from PS3 to PS4 but it felt like a huge upgrade because so many PS3 games were open world and devoid of any detail. If you compare something linear like Uncharted 3 or Beyond to say AC Unity, it might not feel like a gargantuan leap either.
Remember this Beyond gif? It was used to show how PS4 games didnt really get a big boost either.
The desert levels in uncharted look on par with stuff we saw last gen.
Then there is this legendary battle in GOW.
Looks on par with ragnorak doesnt it?
My point is that it was incremental even back then, but we appreciated having insane visuals in open world games that we would typically only expect in linear games. This time around, the gap is smaller but they are fixing things like draw distance, level of detail, lighting accuracy and volumetrics. They dont pop out at you immediately but just go back and compare some early gen games and those incremental upgrades will become obvious and significant very quickly.
You brought up AC Unity. Look at the texture quality and LOD draw distance here. i used to post this shot as an example of amazing next gen graphics. Below that is an ugly screenshot from AC Origins. I think that game still looks stunning but it had these really bad issues with LOD transitions on foliage and just really poor lighting during transitions. You can see the same thing in Infamous and uncharted 4. Stuff like this should be resolved thanks to virtualized geometry, use of RTGI and other realtime GI solutions, and a way better use of volumetric effects that hide any flaws in those transitions. I think Spiderman 2 is a lazy ass game but even that game has done a lot of work on LOD transitions and draw distance despite relying upon baked lighting and normal maps. Avatar, Alan Wake, Hellblade and Star wars to a lesser extent go nuts with detail without sacrificing that draw distance and lighting flaws we saw in so many levels even in the best looking games of last gen.
Here are a couple more: