python said:
an interesting real/actual (not hypothetical as most in this thread) case is Eric Heiden.
the absolute top performer in speed skating became a superb/elite bike racer but never matching his first career achievements by a long shot.
retraining did not work for him and he spoke about the limitations iirc.
i don't think there is a definitive answer. the sprinters fast twitch fibers are largely inherited and theoretically they should lend themselves to any specific trainability regardless of the sport.
in a real world, however, different sports demand emphasis on the development of different muscle groups even in the sprinters who use the same legs. running, for example, requires both the propelling and stabilizing muscles whereas a bike sprinter has the luxury of supporting his weight on the seat and h-bars. depending on the age, it may be too late to retrain.
My initial comments were in regards to top athletes from other sports, making the transition to
road racing, though I realize the initial point was made in reference to the track.
My opinion is that road cyclists especially have not only to be very fit, as are other athletes, but psychologically cut-out for handling the specific type of aerobic pain (which can endure for up to an hour and more, not just a few explosive seconds) and stress (caused by the speed, proximity to others in the field, changing roads and terrain, andsoforth) that the sport places upon the athletes, which is quite different than any other sport with the exception perhaps, I think, of skate technique cross-country skiing with all the due caution such a comparison requires. This is why I don't think NFL running backs and track sprinters have either the specific athletic nor mental qualities needed to perform well in the highest ranks of cycling. And the aerobic fitness required to even finish the Tour makes an NFL game seem like child's play. That's not boasting about cycling, but stating just a simple physiological fact.
In any case, your reference to Heiden admitting the limitations of "retraining" the muscles, though I would also add as equally important, the mind, would seem to affirm what I have always thought: namely that champion athletes are drawn to the sport most adapted to their specific abilities and talents often by instinct, at times through a process of elimination. Certainly there are physiological reasons behind this, but also cultural and I wouldn't exclude that certain ethnic factors play a role (in some instances even a significant one, in others little or none).