I know no case of an astronaut who never flew into space for any other reason than a genuine medical one (or that he retired on his own).

Don Lind waited for 19 years, but he eventually flew. Nicollier waited 14 years before his first flight, he flew 4 times, even spacewalked.
Chister Fugelsang waited 14 years and on his first flight (at almost 50), performed more spacewalk hours than any of those selected with him in 1992.
Astronauts must know that: they will all fly unless something serious prevents it.
There are more than 10 shuttle flights left..isn't there enough room for all those who haven't flown yet? (the last selection should wait until Orion flies).
Even Mullane (ho certainly didn't like Abbey) knew that: they all get their chance.