I have a sister opinion to this take, which is that artstyle is far more important than graphics, and there is a very important difference between the two that is often ignored by large game studios. \*
Your graphical capability can push a hundred million triangles, but choosing the color, composition, and visual coherence of those triangles is more important to how people interpret your world than anything else. Humans have an extremely attuned visual processor that infers so much about the way the world behaves solely by the way it looks.
The reason that classic games can be immersive despite a low poly count is because the artists have made the visuals behave in a way that is coherent with our internal model of the game world.
An example of this is Portal. The portal gun is an interesting gameplay tool, but the game was able to fully take advantage of it because the artstyle of the game was very tightly coupled with the mechanics. They did a great job of making sure that the visual environment offered clues to how the game mechanics worked, which made it so easy for people to quickly grasp how the portal gun operated in complex environments. Had the artists failed, Portal would have been "oh yeah, I remember that game -- the portal gun was a neat gimmick but it felt clunky."
The easy way out is to just try to make your environments and game mechanics as realistic as possible, essentially borrowing that creation of intuition from the real world. But the most creative games have worlds whose completely novel or alien mechanics are coupled with art direction that preserves this coherence, which makes the world just "click." Putting more triangles on screen makes for much prettier art, but the true substance of a game is something completely different.
\* not on purpose; it's just extremely difficult to pull off.