Good thoughts. In addition to the units/weapons in the map, it doesn’t make too much sense to steer the HUD UX/UI icons away from the time period the game exists in (medieval), in favor of a modern day mobile, Win8/Win10 metro-style one.
Plus, AoE is about achieving tasks quickly and efficiently… APM is all the rage these days. How is the new HUD UX better than using classic AoE2-like icons, I wonder? I’d still really like to see the white paper, study, or thesis statement that most every company follows these days, that says making every icon the same color and a flat 2-D icon style is more readable, and leads to a better, more efficient, more intuitive, more user-friendly, more enjoyable user experience for >50% of users 
From a utilitarian standpoint, it doesn’t make sense, imo, to disband the fact that most humans see color and easily recognize variations in color – and also see depth in objects. Making every icon the same color, 2-D flat, and relying almost entirely on unique shapes/silhouettes to communicate their meaning makes the game less enjoyable, as you have have to look harder and study each icon more intensely before clicking it – to make sure you’re finding/clicking the right one. Even if it’s just seconds or micro-seconds more, I bet it’ll happen. How is the absence of color and depth in icons all the sudden more readable 
I made this to demonstrate my point. What if wood, food, gold, and stone were the same color and 2-D (flat) icons?
^-- Looks great, yeah? 
Surely, they’re very intuitive and more readily identifiable and clickable than the following classic AoE2:DE set… 
And even making all the classic icons gold-color is not very intuitive or user-friendly either:
Yet, AoE4 has most every icon gold/yellow everywhere for some reason. I’m surprised the resource icons aren’t purely yellow/gold, and wonder why they get special exception.
I’d be curious to know how decades of UI/UX changes (i.e., ‘lessons learned’) led to every icon being gold-colored and mostly two-dimensional in AoE4. If you like the new icons and think they will be better and quicker-to-read and -use than the old ones, I’d, genuinely, like to hear how you think or know that’ll be the case