In terms of scaling the UI up on larger monitors, that's out of the question -- if it's too small to see clearly, play on a smaller resolution. Increasing screen resolution size in a 2D game just makes everything smaller and lets you see further in the game world, not making what you see sharper. If we were to scale things up that would make everything blurrier.
The UI is not the same as the game-world. I can see people wanting a high resolution world, but also wanting their UI elements not to be microsopic (as they are now).
Then they're out of luck. Unless I rendered all the assets at 2x resolution of what they were now, I cannot express how bad this would look if scaled up. Absolutely, incredibly, terrible.
And frankly, on an appropriately-sized monitor I love that the GUI is mostly out of my way. I switch spells only when I need to, and my health and mana are easily visible right over my character's head. I don't spend a lot of time looking at the maps unless I'm not actively fighting right in that second. So it's really nice because it gives me a really clean and clear view of what is happening action-wise, while still having the GUI there but just in my peripheral vision. I start feeling really cramped on anything less than 1920x1080 these days, heh.
Of course I don't expect everyone to agree. But there's just no way to cleanly scale graphics up -- down, sure, but not up. And fact is I didn't create many of those icons at any higher resolution, so I'd have to redo many of them from scratch. The spells themselves I have larger versions of thanks to the crafting interface, but those are the only ones.
For the rest of it, it's just one of those things where if somebody is of a mind that they want a larger interface to see it more easily, they probably want to see text, their character, and so on in a larger format as well. So just downing the screen resolution fixes that all in one swoop; which is what a screen resolution is for in a 2D game.