I get the message that you will never use a RTOS.
I wouldn't go that far. I can't see filling up the memory of a MEGA or DUE without more structure (in the form of SOME sort of OS.) I doubt whether I'd ever need the "real time" aspects, but that seems to be what is available; I can't see writing my own, especially when the existing rtoses are getting pretty favorable reviews.
Most of my professional career was spent programming under a proprietary, non-preemptive, not real-time OS. OTOH, that company's experiments with real RT kernels was less than spectacularly successful. When your uart ISR starts sending messages instead of just reading the chip, something has "jumped the shark."