I somehow felt that when people are split in two when it comes to their perception of whether it is a happy ending or not, it reflects on how they think about the events of the episode.
I was slightly aghast by the thought of being trapped in a party town forever. Basically the fact that everything is finite is what makes it all the more precious.
Yet when people call it a happy episode, I guess its purely because of the running tonality that Black Mirror has always followed.
Loved your parallel with Fallout. :)