First of all, these developers are people who are living in different locations of the world. Realistically, people are not going to quit at the same time, and they are also not going to die at the same time, so your imagination is quite wrong.

If someone, let’s assume that the leader in the group, decides that they would want to quit, what they are going to do is to announce it that they are quitting from being part of the job and would like to hand over to someone else. It is then a responsibility for them to look for the right person who can take over the job and be able to walk up to the standard that the community would expect from he or she. And if one of the developers should also die, there are others there to replace him or her. That’s just how it works.