Interesting question, I hadn't thought about that before. Moderators are human beings, not robots. So, there will definitely be a time when they will never visit this forum again. I wonder, have the moderators in this forum ever been replaced by other user? Or have the current moderators never been replaced since they became moderators?
Why will you want to replace a moderator that's doing his job perfectly and has dedicated his time to the job. The position of a moderator is a very sensitive one, and you don't have many people who are qualified for this job. Some who are qualified don't want the job. The position of the moderator is not something that has tenure duration that you will want to change them periodically. If there ever come a time when a moderator don't visit this place again and there is need to replace such a moderator that has gone offline for too long, the administrator will change such person if he sees someone that can fill the gap and do the job very well.