I enjoyed Thanksgiving day with two large community families. Somehow this got me thinking about the opportunity costs of missionary work. I've had a related thought before, but I realized one could consider the opportunity cost of missionary work literally in human lives.
Put simply, one can reasonably expect to have two children in five years. That's an average and somewhat arbitrary rate that I made up because it sounds reasonable. Clearly some people can't have two children in five years, and for them missionary work makes sense. If you know you can have two children in five years, then you have to convert more than two people every five years in order to overcome the opportunity costs.
It's not insurmountable, but it is important.