Well, it really is quite nuanced.
First things first, texture, shadows, lighting, post-processing and such are pretty resource intensive. Still a reliable way these days to get a few more extra frames per second is to lower shadows.
So already, more powerful hardware is something that's always going to improve, as computer power improves.
Second, storage. Yes, a lot of games these days are physically larger, because all of those graphical improvements cost resources, and one of them is the game is larger. This puts a serious bottleneck on how it's stored. This has always been an issue with storing media, and that issue is likely not going to go away anytime soon.
Third, finally, is art style. Whatever we experience as consumers as a game is essentially a compromise from the original artistic vision of the developers. Artistic vision is going to influence the graphics of a game. Some types of art include dynamic lighting, which dynamic lighting with deep shadows, even if it's not photo-realistic, might turn out to be surprisingly resource intensive.
For example, sometimes certain artistic elements just can't transfer over across mediums. Something can look incredible as a painting, with subtle brush strokes, blends, and super light coloring, and just look dynamic. But let's put that same thing as a tattoo and.... it's not going to age well on someone's skin. So either the art has to have some license to be altered somewhat to work as a tattoo, or you'll want a different piece of art that works better for tattooing.
How these three things interplay is what we always see with game graphics. Computer power improves, which is both a blessing and a curse, as this can free artists and game designers to put out content closer to their original artistic vision, but it might be surprisingly resource intensive, and may be so large as to necessitate multiple discs to store it, or multiple GB's if it's digital.
Just my two cents.
