Of course discussion is about mutual understanding.
My problem with your theory is these, in my opinion major, differences:
1. the system: as others have already pointed out before, there is a crucial difference between the system of the UdSSR and the one of an open-source project generally or specifically FlightGear. Upon others, there is no central institution of government and thus no guideline on what "should be done". Also, within reasonable borders, everybody is able to speak out his or her mind freely. Additionally, nobody is forced to do anything at all (basically)
2. what happened: as far as I understood your history lesson
basically this happened in the UdSSR:
*) the government wanted to fly to the moon, so they instructed different design bureaus to develop concepts
*) different engineers couldn't agree on one solution, the concepts were, as it was called, "shotgun married", which didnt work out. Additional factors seem to have been a lack of funding among others
Now, as I see things, already on the first step we have a major difference: There seems to be little to no interest in FG to fly to the moon - that's it.
3. the reason things didn't work out (personal opinion):
* for the UdSSR, as mentioned above
* for FG, well, there is noone who does it. There seems to be at least one person, you, interested in using it for whatever reason, but you didn't implement it and thus it doesn't exist. I have yet to see many other people at all that are interested in it, let alone capable developers interested in it
About the facts you stated above:
1. I, with my limited technical knowledge of FG's internals, agree that it seems to be possible to modify/adapt/expand FG in a way that it could fly to the moon - or an open-source space simulator could arise
2. Mankind could have "put a man (woman as well) on the moon" again, given that it already happened I think it's safe to say this is possible
Thinking about it now, I actually do see a similarity between both, to me the basic reason why neither has happened yet is the lack of interest. I don't mean the general lack of interest, there are for sure many people excited about humans going to the moon for whatever reasons, but the lack of interest where it matters, i.e. on the scientific side (as it stands, there doesn't really seem to be a reason to go the moon at all) and as well not a sufficient interest in the general public to elect politicians who as an agenda push "hard enough" on making a new moon program happen. Looking at this from FG's side, lack of interest in those that would be capable of investing the required time and coding the required code seems to be the central reason as well.
I'm really interested in hearing your thoughts about this.