Its perfectly possible to have realtime requirements and still have variable latency as long as all the variability is bounded. Think video playback or gaming,

Its perfectly possible to have realtime requirements and still have variable latency as long as all the variability is bounded. Think video playback or gaming, both of which run on traditional OS's because they tend to be fast enough to meet the frame deadlines. Of course as we all know, the requirements aren't "hard" so dropping a frame/etc is a non critical failure. So, there is a spectrum of realtime and how "hard" it needs to be. There are a lot of cases where simply having a real time OS on a traditional processor is fine as long as everything can be sufficiently bounded.