Unsuitable, how? Solaris has had realtime capability for fifteen years, and most recently has been the embedded OS of choice for Sun/Oracle storage appliances.

Unsuitable, how? Solaris has had realtime capability for fifteen years, and most recently has been the embedded OS of choice for Sun/Oracle storage appliances. plan9 not being Unix (at all) nonwithstanding, Coraid's AoE products have used plan9 as an embedded OS in the past. Certainly in the recent past, US destroyers had embedded signal-processing systems built by a well-known HPC vendor running a derivative of INRIA ChorusOS. Green Hills Software's Integrity RTOS && development environment are also quite commercial, and have many successful deployments. Any insinuation that military or embedded applications should be Linux (or Windows) deployments, or even necessarily free software, is ahistoric and, IMO, under-informed.