I may be in a minority here but that simple UI/UX design is the one of the main features that gravitate me towards stock android. I like the simplicity of that UI and all other themes look like chaos on my screen (that's just me). One point that I agree on is Google should definitely add useful features in their stock android. The sweet point is something like Oneplus's OxygenOS. It is close to stock android with added features and some UI tweaks (though they're going off that route lately). But, from UI perspective I think it is atleast top 2 for me (other one being OxygenOS).