https://www.google.com/search?q=flat+UI&tbm=isch.
When I say "flat interfaces", I mean GUIs characterized by
- Monochrome widgets/elements with no shading/shadows.
- No clear indication between what is and isn't clickable.
- Artsy 'design' being prioritized over efficient workflow.
- The same overbright/washed out colors everywhere, the ones you can get from http://www.flatuicolorpicker.com/ (no, I'm not making that up)
- Redesigning the UI every couple of months for the sake of change.
(You also get infinite scrolling webpages with important information at the footer yet you're never quite able to reach that. Smart!, but that's besides the point and applicable to web design only.)
Meanwhile, if you look at Windows NT 4.1 from over 20 years ago, you see this:
- Buttons have a pseudo-3D effect with a bright/dark side that 'pops out', giving immediate sense of interactivity
- Consistent layout between programs with the menu bar/entries below the window title, status bar at the bottom of the window
- Sub-sections of windows are shaded, like the buttons and windows themselves (compare Windows 10 notepad vs NT 4.0 Notepad!)
- A smart use of numeric control inside natural language sentence
- (Efficient rendering!)
(I still use the Windows 7 'classic theme', by the way.)
Now I ain't saying Factorio should look like the Hotdog Stand theme for Windows. In short, I'm simply saying that:
- Factorio's user interface works, people are now used to it, I see no reason to overhaul how it works.
- The UI works well by contextual hinting with a sort of digital-skeuomorphism
- The UI also has that shaded (what some call "ugly") look to it, but it's efficient in transferring usage hints and I think a nice throwback to days gone by. (I personally like how it looks.)
- Resources spent overhauling the UI can be better spent making the game itself better.
These are just my $0.02, request for comments/thoughts I guess?