ThreadX has been around for 25 years. You licensed it and put it in your embedded system. It's in a lot of products.
"currently running on over 12 billion devices around the world"
Is this like another baseband processor?
Everything from your x64 CPU to microSD to credit cards(not the readers; readers run antiquated Android and soon known-bad Chromium) runs some form of weird slow proprietary RTOS. It is what it is. I bet it takes ~century with help of superhuman AI to make those run an open and verified code. The situation is improving too, slowly, because using buggy proprietary code is not a goal, but means.
It's ok to be disgusted about the status quo, but that is not necessarily worth your time; IIRC one of original complaints by RMS on the state of software freedom that lead to Free Software Movement was about some HP printer running buggy custom OS. Even the point people said enough is enough goes back that far.
> weird slow proprietary
They're often not weird, a simple single task runner with a few libraries to handle common tasks and cryptographic operations. Very simple, lightweight, and they generally share a common high level architecture (there's not much variation in an RTOS)
They're often not slow, they're minimalist OSes - barely qualifying as an OS if at all - designed to run a single task, with time guarantees, and to get out of the way. In fact, if it's a single task you need to run, they're faster than any general purpose OS - by design!
They're often not proprietary - a handful of RTOS with huge market penetration used in billions of devices (and now ThreadX) - are open source and have permissive licenses. What IS often proprietary about them are BSPs, but that's a whole separate issue. Yes, there are a lot of proprietary ones out there, but as a blanket statement, it's simply not true.
> readers run antiquated Android
Many use a stripped down version of AOSP, which has become a de facto standard BSP, yes. But many, many others do not (usually a flavor of embedded linux, or an RTOS).
> about some HP printer running buggy custom OS
It was a Xerox printer, and it was because he was frustrated from adding existing job management and notification features he had written to the new printer.