You say this almost like we sit and daydream while looking at the AoE2 icons 
Take a break from the game and admire the artwork, so to speak.
On the contrary, having icons that you can actually see what they are at a glance means less time in the UI and more time playing the game. Less time trying to figure out what the icons are and mis-clicking.
When one gold icon shows a wooden gate and another gold icon shows a stone gate, how is it easier and more intuitive to distinguish them that way than if you saw a wood brown for one and grey stone for the other? For towers, the same thing.
And if you’re wanting to create military units and wish to know what they’ll look like before building them, how is a simplistic gold silhouette statue icon with one, two, or three dots next to it better and easier to use than actually seeing a representation of what the unit looks like? And same for every civ, iirc
With AoE4, it feels cold and impersonal. I blindly click gold icons for units, buildings, and tech upgrades just to click them in a whack–a-mole fashion because they illuminated and I can afford them. From that perspective, I spend less time in the UI because I don’t want to sit there trying to decipher every gold icon. But yet there are other times when I spend time just trying to make sure I click a lumber camp instead of mine, and stone wall instead of wood one, etc., or to, indeed, read all about what the icon is because visually it’s not very meaningful. In all cases, it’s far less than ideal, imo
I hope they are making classic-style icons that can be enabled. Then, you can enable the modern day metro mobile phone app icons if you want, and others can have the more classic, era-immersive, civ-unique, and visually informative UI/UX