He probably gets beaten by a younger fitter horse and his stud value plummets. OTOH many race horses have a long and distinguished racing career and later went on to stud, or become brood mares.
It all depends on luck and chance in horse racing. We can all study form and try and guess how fit a horse is, but on the day punters and owners alike are in the lap of the gods.
I don't know too much about the Triple Crown, but I do know that it would have to be an exceptional horse to win the Grand National twice (though it has happened), and I wouldn't risk my money on a previous winner. The Derby is another matter, but then it's a flat race.