Why anyone thinks that displaying something like a desktop OS with many static elements is a good idea is beyond me. Yes you can use some tricks to reduce the impact but why should you adapt in this way? I also realise that OLED manufacturers have dealt with burn in to a degree. However, it's not eliminated. The future for PC screens is probably going to be microled, not OLED and not LCD. However, if you're the kind of person who replaces monitors/laptops frequently, maybe burn in is never an issue you will experience.
Preempt is all about interrupting processing in favour of other processes. Unfortunately I was not able to find a good page to explain this after a quick google.
In practice, using RT for a desktop would probably mean performance would be slower, not faster. The overriding purpose of an RTOS is reliable and consistent processing time.
If God is perfect, why did He create discontinuous functions?