Simplex RT works fine. We are all trained how to listen before using the PTT, how to formulate a transmission and how to allow space for an answer - most of the time.
ATC works very well. All efforts are made to avoid any confliction from the flight plan stage to the tactical stage - most of the time.
Pilots develop an awareness of other aircraft around them by listening to the RT, watching TCAS / Wx radar and looking out. This is most useful in understanding complicated situations - most of the time.
Even when the odd situation occurs where time and ideas run out and things are tight, people get away with it - most of the time.
But - not all the time. The Tenerife crash for instance involved a double transmission. One aircraft in extremis stuck on the foggy runway, trying desperately to stop the other rolling, while a tower communication took place and the RT was jammed. The question is, do we want to plan for these rare last second occurrences? I would guess yes - same philosophy as with GPWS and TCAS. We should have a means of communicating where an priority transmission overrides other radio traffic. There will be many who can explain why this can't be done, impossible, too expensive or unworkable. I am not here to explain in this modern and digital World how it should be done but you young bloods should suss out fairly quickly how it could be done. (spoiler - don't forget the open guard box)