How can you wonder? Isn't it obvious?
Graphics that would leave us impressed in 1985 or 1995 or 2000 are considered nothing to write home about a few years later...
While the graphics of Super Metroid might not be anything spectacular, the pixel art is phenomenal, and replaying it recently I gained a whole new appreciation for how future-proof the art direction in that game is. I think there are many more examples of this out there, too. For instance, people often bemoan early 3d games (Nintendo 64 games especially) for aging horribly. On the whole, that is true. But there are certainly exceptions, such as Majora's Mask, where (again) the art direction saved the game's graphics from the awkward low-polygon limitations of the system.
IMHO, for 2D, that was achieved with the NES generation. And for 3D, with the GameCube generation. (My son fired up Metroid Prime yesterday on the Wii, and it took me a while to realize it was a GC game, not a Wii game.)
But, yeah, Super Metroid is amazing. I only played it recently (~6 years ago), never the original, and I still enjoyed it thoroughly.
I do agree with you in general, though. Which isn't to say that graphical advances aren't still perceptible (compare SNES-era sprites to any of the positively gorgeous sprites from Odin Sphere or Rayman Legends), but at some point you cross a threshold where graphics cease being cringe-worthy and start being merely dated. FFVII is a cringe-worthy example, whereas The Elder Scrolls: Oblivion remains breathtaking to behold (as long as you don't look anyone in the face, that is).
Also they got a bit of help from kubrick ;)
I'm particularly curious about the nature of that 'expectation evolution', and how exactly that works- and how, for example, people not exposed to the earlier work might interact with it.
I'm reminded of an interesting story of a father walking his son through all of video game history: https://medium.com/message/playing-with-my-son-e5226ff0a7c3