You would think that we would only see better graphics from there
Strict graphical improvement from one game in a series to the next is hardly ever a hard rule.
Even when systems get replaced by systems that most people call "better", there are often particular cases or respects in which the old system has benefits.
Like, the lighting system in games like the original Doom is in some respects very crude compared to later lightmap approaches; it's only able to apply flat light levels across big chunks of geometry, and can't handle stuff like the smooth radiosity gradients that a lightmap bake gives you. But it trivially and efficiently allows for, say, sharp hard environmental shadows.
But oftentimes things just plain get downgraded. A lot of the "improvement" that happens across a generation is developers deciding which things to prioritize, and which things to slash.
Sometimes this can even persist across generational leaps. Halo 5 almost certainly doesn't have a system analogous to Halo 3's "area specular", since objects in the game suffer from tons of anomalous rim lighting.
Changing game design can also have a huge effect on what you can and can't get away with.
Even weird stuff like tweaks to camera acceleration and whatnot can have a big impact. It's amazing what FFXIII gets away with just because it has a camera that's very difficult to parallax without rotation; there's tons and tons of giant pre-rendered cutout background objects, which would be very obvious if it wasn't so hard to stop the camera from swooping to align with your movement direction. (Probably a big part of why FFXIII-2 "has worse graphics.")
Developers usually try to make each new game get perceived as "more graphically impressive/modern/whatever" than the last, but whether that works out in any given case is ultimately subjective...
Thanks for the insight, as usual. Why aren't those walkable objects in Duke Nukem 3D stackable? Could you stack them and create a perfectly convincing true 3D perception? Like playing a completely vertical level from platform to platform...
I'm not very familiar with their functional constraints, but at the very least there'd be a performance cost associated with them.