And the Pizza is not the Sauce or Cheese.
There's also the question of _what_ is being simulated. A superhero game which uses hard-realistic physics and biology wouldn't be a superhero game. It might be an entertaining setting about humans with unusual equipment or strange mutations, but it wouldn't allow players to have the experience of being one of the X-Men or the Justice League.
I think it's useful to replace "realism" with "fidelity." Is the game "faithful" to its source material? Then it doesn't have to be literally "realistic." Note, however, that pretty much all fiction or fictional settings include a lot of basic real-world "reality" in their fantasy -- even in a shonen anime, gravity points toward the floor, wood burns, the Sun rises in the east, etc.
So "fidelity" should start from a position of literal realism but understand where it differs -- and ideally should make those areas of difference explicit.
Ah, diminishing marginal returns! The graph that rules the universe!
This is something I've been crowing about to largely private audiences for years, and it's what I've been slipping into my homebrewed systems - if only for my own sake - for just as long. You just can't have a good system if it's not rooted in a firm substance.