The original GUIs were all flat because that was the default. A button was a rectangle with text in it. A checkbox was a rectangle with or without an X across it. Pure black on pure white, no colors or shades. Windows used this style until Windows 95.
replyNobody seemed to have a problem with it. It was largely clear what was a button and what was a checkbox. In hindsight it was certainly uglier than the 95 style (maybe just because I grew up with that) but it wasn't unusable at all. As you say, it was clear what was a button, what was a checkbox. I think it was because GUIs were mostly made out of standardized elements whereas today we have everyone trying to put their unique spin on every element.
I might be misunderstanding what you're saying, but: In the PC space, Windows 3.x definitely had some skeuomorphic elements. This presents most-commonly with the minimize and maximize buttons[1].
replyWe have to go all the way back to Windows 2 in before we find flatness.
And Windows 1 and 2 were barely even used. Windows 2 sold less than 2 million copies. Windows 3 and co sold 7-ish million. Windows 95 sold more than 40 million.[1] There's a lot less people to complain about Windows 1 and 2, and those people were probably a lot more experienced in the first place.
reply[1] https://web.archive.org/web/20220418124401/https://techland....