You are confusing good and readable with beautiful. A pencil sketch can be readable and good, yet still look like it can be changed. By contrast if you make it look like perfect it will look like it cannot be changed.

Sorry, I don’t understand the point you’re trying to make here. Finding ways to present information that are both functional and attractive — and stay both of those things as the information evolves — can be a challenge, but I don’t believe those are completely independent axes and I don’t think achieving and sustaining both in combination is an unrealistic ideal. Indeed, I’ve spent a significant part of my professional life creating specialised user interfaces for various products that try to do exactly that.

The point is if something looks too polished people assume that polish is all the way. You look at a polished chart different from a sketch. If I show you a napkin sketch of some new idea I have you understand it is new and not well thought out, thus you ask is the big picture idea correct. If instead the same idea is well polished in full color you think the big picture is correct and start looking at details that might need some adjustment. Depending on what sort of feedback you want, you need different types of drawing.

This is true, and why some diagramming packages include "draft" modes that intentionally change the presentation to make it look less permanent/polished looking.