Major props for separating the mission from the vision and going with the mission.
I tell myself (and whoever asks) that I'd be okay if our startup turns out not to be viable, but I'd never forgive myself leaving our potential as a big fat question mark. We finally started a beta last week so answers will come in time, I just hope that if it ever comes down to it, we also pick mission over vision.
Best of luck with the next incarnation of Referly.
I think a tougher question (for me anyway) is when you have moderate, sustainable success, but not the stratospheric rise that you envisioned at the outset. At that point, the decision becomes tougher because walking away means that you are giving up something of significant value. I have also found personally that I like building businesses, but running them? Not so much.
So, the business becomes an anchor, keeping you from a greater vision/mission. At some point, you finally realize that it has to be sacrificed in order for you to get back on track, but precious years can be wasted under the illusion of success before you get there.