Let me start this by saying I don't know either from experience or from hard rules. Here is the way I think about the situation you posited -- due to operational issue of carrier and no fault of your own, you miss the flight.

I think of an analogy to help me. Imagine you booked a very desireable reward flight and the connection is missed because your first segment is late late late.

If there is no award inventory for the next month on your next leg, would it be reasonable for the airline to make you sit in that city for 30 days until the next exact fare bucket opens up? On the other hand, would they look for a reasonable routing, perhaps looking for a differnt bucket like Y or F and get you onward?

Based on all the experiences I have had with airlines, the second case is much more likely. Remember, I am not talking about customer requested changes or customer no-shows, but that operational issues cause you to miss the booked segment.

Going with the logic of the analogy, I would suspect that the airline would look for a reasonable way to get you to your next destination and not insist that you conform exactly to the flight, date and routing of the ticket -- this is not a voluntary change on your part. They might have to use some irregular operations ticketing magic, including the code for disrupted flights, if the systems won't allow the extra segments to be directly entered with your eticket, but they will find a way to get you moving forward as soon as a seat is found. Might be a case of downgrade versus longer wait, but in that case they would likely discuss it with you first.