I feel like the old icons were perfectly fine, and the developers should have just let them be.

Some of the new ones are good (for instance: the lamp and object icons), but they are much harder to read, and they lack character/aesthetic. The reason why monochromatic icons work in other software such as Photoshop and Maya is that their interfaces use bigger icons, and there aren't such huge amounts of them crowded together like Blender does in the modifier selection menu and the mode-changer-thing. Having colored icons makes it MUCH easier to locate icons quickly, and they just feel more natural.

I think that monochromatic icons aren't necessarily bad as a concept, but they don't work well in blender. I also want to mention that they are pretty much the only part of the main interface that hasn't been changed in 2.8, so it will make it even harder for people who are used to 2.7x to switch over. Nobody wants to relearn icons.