Embedded for sure eats the world. If you're looking at that then QNX, FreeRTOS, and similar options are also big in the running. The thing is if you're targeting a particularly well-defined piece of hardware and application, and you know you're going to want to customize and optimize for that combination, then you're generally going to be better off with a smaller, simpler starting point than something which is designed to run on pretty much everything and for almost any application. The different licenses complement that, but I think even if the licenses swapped the design and goal difference would affect things more.