I've wondered the same thing. Personally I think the chances are slim given the recent announcement of LAX-FRA service.
One thing that is somewhat unique about SNA is that a lot of travelers who live close to the airport will drive to LAX to fly a transcontinental or international trip, mainly because total travel time is quicker than a connecting flight.
What this means is that launching nonstop markets from SNA ends up cannibalizing service already available from LAX. I'm fairly sure this is why AA discontinued SNA-JFK (besides the fact that the transcon market was flooded with capacity at the time).
Going back to SNA-IAD, I don't see it being especially feasible for connections to Europe. Because of the polar route, LAX-FRA-XXX is going to be quicker than SNA-IAD-XXX, even factoring in travel time to LAX. I also think UA wants to avoid cannibalizing SNA-DEN, because SNA is a key feeder market to support eastbound connections there (remember there is a big battle for market share in DEN between UA/F9/WN).
That all said, I could be wrong. But my guess is new feed into IAD will be from cities that can benefit from the transatlantic connections.
What do others think?
I think your reasons are sound, but . . . UA had the nonstop IAD-SNA open for sale, meaning it was viable at the time. I think there is more like a 50-50 chance UA would find good enough reason to do it again someday. I don't think DEN-SNA is one of their concerns. But you're right, LAX-FRA definitely reduces the need.