1) It’s just a post-processing instruction; generally, it doesn’t physically change anything about the picture taking process, unlike shutter speed and aperture.
2) While the term “ISO” refers to a real standard, camera manufacturers don’t comply with that standard, like, at all. Many cameras are more than a full stop off the proper ISO. For that reason, light meters simply won’t work as you expect them to. Using the same settings on multiple cameras will yield very different results.
3) High ISO could be eliminated completely; it’s just an arbitrary limit to a multiplication problem.
4) Low ISO could be eliminated by simply using image averaging, allowing us to get long exposures in-camera without using an ND filter. This would also reduce noise in good conditions.