The Nokia N900, running Maemo, also supported multitasking in 2011. It was just toppled by a similarly dedicated team of executive fuckups.
That's why those OS were mostly used by geeks and power users, and "regular" users were using feature phones that "just work".
One of the strength of iOS and Android were to create a completely different userspace that what we had in desktop OS, more adapted to mobile. They combined the "just works" aspect of feature phones with the power of smartphones.
Anybody could run a full multi-tasking OS on a mobile device trivially. The performance sucked and you killed your battery super quickly.
The innovation was in multitasking that didn't result in a terrible user experience, and it took a lot to get there! And the answer wasn't "welp what if we just treated this thing like a desktop".
And it's still not a fully solved problem - there continues to be a lot of movement around how apps are defined so that they can be efficiently concurrent! (or at least give the appearance of concurrency)
And the UI did have plenty of affordances. Basically all the apps were custom, and I vaguely recall there being something close to the home / back on screen button android used in the early days. Heck, it's still a pita to switch apps on my Pixel: swipe up, but not too fast, or it'll bring up the full app list instead of the switcher.
But sure, there's plenty to dislike about the n900: it had a resistive touch screen and a stylus. Turn by turn navigation sucked for most of its life. The app store launch was so botched that it was basically dead on arrival. The microusb port sucks.
The N9, N900's successor, shipped with MeeGo 1.2 "Harmattan" and had the most simple and elegant UI I've ever seen on a mobile. The phone-UI combination was a masterpiece. But it was still Linux, with all power-user features under the hood.
Windows Phone 7 moved to CE 6.0, then Windows Phone 8 to 10 were NT based.
Wikipedia says Windows Phone 8 was released October 29. 2012, which is around the time the ARM-based Surface RT was also released. A significant event for Windows NT to be on an architecture other than x86.
Yeah, I too liked to run Windows NT 3.1 initial release on my DEC Alpha and MIPS workstations. Wait, what?
(I think you meant to say that the support for ARM32 specifically in Windows RT and the NT-based Windows Mobile 8+ was a noteworthy milestone, which I suppose is a fair point.)
Despite that, the phone sold several million devices and people were paying huge premiums (often $200-400 over price) to get it shipped from these secondary markets to where they lived.
The demand was there and Elop decided to kill it anyway. He also never released the second phone required by their Meego contract with Intel as I recall.