My Endland campaign recently lost a player due to lack of free time. With only two players left, my immediate reaction was to retire the campaign because most of what I had planned would have involved three players. Looking for new players...not really something I want to do. So far I have been lucky with my RPG groups, but I don't really relish the thought of introducing a new player only to find out that he or she is that guy or a variant thereof.
But after some time to think about it, I decided to run the campaign anyway. Every time I looked at my notes, the world drew me in again and new places and characters evolved. So I sorted through what I had planned and found that a lot of stuff worked just as well with two players and the rest - I'll just keep it in mind. There's nothing that has to happen at a particular time. We'll see how it all works out with two players. I want the campaign to continue very much, but if it turns out that two are just not enough, I'll retire it for the moment.
If anyone had told me that I would be that addicted to world-building and GMing a year ago, I would have laughed at them. I would not want to do it full time, I like playing too much, but I would not want to miss it either.