GUI computers conquered the mass market in the mid-to-late 1980s; by the 1990s, Microsoft caught up, and the x86 PC nearly wiped out everyone else. Only Apple survived.
But in the late 1980s, the pre-Cambrian explosion, there were 4 main players:
• Apple, first with the Lisa, then the Macintosh, running related in-house OSes
• Commodore, with the Amiga, running an in-house OS built using some parts from academia
• Atari, with the ST, running an OS from former CP/M giants Digital Research
• Acorn, who stuck with 6502 longer than anyone, then did the mother of all 2nd acts with the most successful RISC CPU of all time, which outsells x86 100:1 and powers the Mac I'm typing this on.
They did it by taking their original 8-bit OS, modernising it for a 32-bit CPU, and putting an astounding GUI on it which influenced NeXT, Microsoft, Apple, Android, Linux and everything since.
People often wonder what if the original MacOS was open source, or if Microsoft open-sourced 16-bit Windows, or IBM open-sourced OS/2. Amiga and ST fans wanted FOSS versions so badly, they rewrote the entire things from scratch.
Well, here is the worked example.
Acorn's OS is still around, still maintained, still runs on modern hardware made and sold cheaply today. It can talk IPv6, has modern web browsers and email and chat and media and all that stuff that people said Linux and BSD needed for decades, and it always had them.
And it's FOSS.