Gosh, there's so much wrong here.
> 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.