There are not any real time patches for Linux: there's only the low-latency kernel. Low-latency and realtime are different things. A lot of the time the two can appear superficially similar, until they don't and your airplane falls out of they sky.

The realtime executives are not operating systems. Executives and operating systems are two different things.

QNX is a realtime operating system. It addresses the niche of a realtime operating system that isn't being met by anything else out there, and has been doing so for 40 years, even if free versions for hobbyists haven't been available for many of those years. It's not about market share since that's always been there. It's about mindshare and given the comments here, that's obviously missing.