And, no, there's nothing implausible about the idea of reproductive difficulties gradually causing a species to go extinct. Just check out the difficulties scientists have been having getting the panda species to reproduce.
Exactly. Humans only need to have two children per couple to replace themselves. Assuming ever Andorian can find three suitable mates, they need four offspring per group
just to keep the population stable.
Prior to the genetic disaster that occurred in the past, this was not difficult.