Where you especially lose me is when you insist NRP should be viewed as a game as compared to a story. No. People abandon games as easily and for as fickle reasons as they abandon stories. People can do it both ways and both have their successes and failures. The Guild (and other places I'm sure) tends to err towards the story end, since as already stated, people who want to make a win/lose game out of it can go play Civ and other things. There is absolutely nothing wrong with this.
I've quit videogames for as simple a reason as the fact that I recognized I was no longer in a position where I could win/achieve my goals. Time for another run for that achievement, since this run is busted. Now imagine that I have a roleplaying videogame, where I am expected to put in x amount of time to work on my replies, and I would be doing it for something I have already realized I cannot win, achieve, or otherwise get some sort of (invented or not) victory condition out of, I would be quick to say that sorry guys, I've lost interest in the roleplay, best of luck. And that supersedes NRP like you said, because there are plenty roleplays (narrative or gamey) that have had that happen
just from my own experience. And this element of no longer having a victory condition is present even in narrative RPs, because different people have different expectations and goals for themselves, their characters, or the roleplay. Once those goals can no longer be reached or they are so far out of reach that it would take immense amounts of work (and a willing GM to potentially cut you some slack), what is the incentive to keep playing? Players that derive their enjoyment of an RP out of hitting checkmarks and achieving victory conditions will no longer enjoy the RP, and leave.
The thing is that investing time in CIV is nowhere near as exhausting as it is to write a roleplay post. All I have to do is sit there and click buttons. It's no wonder that when a game mechanic gave another player the victory, my enjoyment of the game disappears, because all the work I put into the game thus far can be, or has been nullified by that singular loss. 8(