If it's a detail that you have to pay more attention to see, the diminishing returns are already in place. The last huge jump was between ps2/gamecube/xbox to x360 and ps3, especially comparing the later gen to the beginning. Sure, there have been leaps and strides in a lot of areas since, but we're getting to the point that the major developments are less in generically making things prettier and more into investing a lot of time into making very specific physical effects realistic, ones that could potentially just be cloned with baking them in rather than real time. A lot of the reason for these diminishing returns is that moore's law is simply no longer in place due to major constrains in shrinking hardware anymore. That was unavoidable. Anyways, my point isn't that you cannot tell the difference between x360 to xbone or ps3 to ps4, but that when comparing a good late gen game on either the 360 or ps3 versus a game even late into the more recent gens, and there aren't a whole lot of improvements.
We're getting into "glass half full/half empty" territory here, it all depends on what you're personal standard of a "leap" is.
I'm not saying 7th gen to 8th gen was a quantum leap, no, of course it wasn't, there are diminishing returns, I just think people overhype that a bit, but maybe times really are changing?
Basically if I'm going to get real nerdy here's how I would break down the evolution of graphics over the last 20 years, first the focus was on textures, with generally low level (though higher level than before of course) geometry, the original Max Payne is a good example of this, low level geometry but still some snazzy textures, this "texture is king" era culminated in Valve's Source engine.
Then in the 7th gen and early HD era the focus was more on geometry (ie what they used to call "bump mapping" or real mapping) and fancy effects like bloom lighting or the "stuff looks wet" effect which was a real "whoa" effect at the time (think in Gears of War 1 when it's raining in the woods or water trickling down the stairs in Bioshock)
Trouble is oftentimes they had to sacrifice basic texture work so sometimes, although this was less of an issue on PC, you'd have some low level textures here and there, worse than some 6th gen games' textures, this was why Valve was able to still get by with the Source engine just fine during the 7th gen despite a few small updates.
Now ever since the 8th gen it's been the best of both worlds, all the fancy effects and geometry of the 7th gen with the return of sharp as hell textures, sharper than anything seen in the 6th gen, I think that's a significant leap, but now is when we are going to get real diminishing returns because we've already pushed things very far, now we're getting into weirder territory like ray tracing, which is neat, but a little overhyped.
I remember when I replayed Infamous: Second Son in 2018 how great it still looked after 4 years, it barely looked dated at all and that was a "launch window" game, I've definitely been not super impressed by what I've seen on the PS5, I did say we're still early and not there's not a clear indication where it's going yet, but Infamous: Second Son was a "launch window" game and still held up pretty well compared to the later PS4 games, it's not like comparing launch window PS2 games to later PS2 games at all.
