The argument for restricting the use of color/animation/sound in UI elements is that you don’t want to have the icons distract from the content.
The problem with that is that, sometimes, the user is looking for UI elements. I don’t see us build an AI that detects that and makes icons more colorful/distinctive (and don’t even know whether that would be a good idea), so I guess the ideal is somewhere in-between: icons that are neither too bland nor too colorful.
This justification makes no sense to me. Icons aren't window dressing, they serve an actual functional purpose (identifying things that you can click on to perform actions). Removing color and making them all look similar makes them worse at their intended functional purpose.
The problem with that is that, sometimes, the user is looking for UI elements. I don’t see us build an AI that detects that and makes icons more colorful/distinctive (and don’t even know whether that would be a good idea), so I guess the ideal is somewhere in-between: icons that are neither too bland nor too colorful.