The bottom line is that its extremely hard for any trainer to stay in the business for an extended period of time at major tracks on purse winning %'s alone.
What trainers really build their finances or retirements on are commissions from selling a horse(Most trainers receive 5-10% when they sell one privately) and moreso on breeding rights in stallions or the 5-1% they get from the owner if the horse is sold as a stallion(most owners do pay this, not all(like one guy I know), but most).
Guys like Zito, Pletcher, Mott, Baffert, etc build up these breeding rights which can be sold each year or used.
A guy like Zito has a breeding right to Unbrdidled's Song(trained him at the end of his career) and each year he can sell that on the market or do a foal share with someone who has a mare and then race or sell the foal.
Guy like Pletcher will soon be making more money on his breeding rights than he will on the purses, and when you sell a horse like Ashado for over 9 mill, its customary to give the trainer who trained the horse 5%.
If you train a horse that the owners sell outright as a sire, its customary to give the trainer 5% of the money received.
Simply living on purse money at the highest level of racing is a non stop rat race thats hard to take.