Say a typical captain flies 250 segments per year. (Some fly less, especially since the more senior ones get to pick their routes and often choose long-haul international.) And say s/he works for the airline as a captain for 20 years. That's one retirement in 5,000 flights.

If a very frequent traveler flies 100 segments in a year, that's one chance in 50 of being on a retirement flight that year. Most FTers don't fly that many segments per year, but do fly for several years. The chance of seeing one of these over one's flying lifetime is probably a few percent.

(Feel free to substitute your own numbers, but I think the approach is valid.)