I think that more recently all 3 of these riders have hinted they'll be back for another crack next year, so this is an unlikely scenario.
I think that if one of them goes, Tomicek gets a place as he came 4th in the qualifier. If any more of them retire i suppose that IMG would get an extra pick or two.
As i said though, i don't see any of them stopping. And if any of them do, i certainly doubt it will be more than 1 of them.