> You could - with extreme effort - make an Atari ST multitask. And of course with the Amiga it was built in. ... So why did we throw that away and go backwards to DOS
Mine is a US-centric perspective, it but always seemed like Commodore and Atari were selling niche follow-ons to their eight-bit lines, with momentum heavily toward DOS, even in the second half of the 1980's.
Open up the back of a 1987-era computer magazine, and the pages are full of companies that will sell you a DOS machine, expansion cards, operating system, etc. All these participants in the market are sinking money into the platform to try to establish a competitive advantage by making something better. (The Compaq Deskpro 386 and various video cards come to mind immediately as places where the market rapidly outran the original developers of the platform.)
But if you want an Amiga, you're stuck with Commodore. If you want an ST, you're stuck with Atari. Two individual companies both trying to build an entire _platform_ - custom hardware and OS - that's competitive with the collective output of an entire segment of the industry. I guess it's easy to say this now, but both of those companies picked a hard battle to fight - and both did very well, all things considered.
> ... single-process Windows - which eventually reinvented multitasking nearly a decade later and sold it as if it was the most astounding development in the history of computing?
The development wasn't the technology, the development was getting that technology into a delivery channel where it was relevant to a large group of people. This is why Windows 3.0 was such a big deal - there wasn't anything technically novel in it, but it was cheap, worked with what the customer base already owned, and attracted enough investment to overcome its (many) faults.