Elite fighters that switch trainers, some of them do better ala Shane Mosley but it seems most of them either stay about the same or do worse, ala De La Hoya. Mike Tyson is probably an example of a fighter who needs someone like Atlas or Rooney, people he doesn't necessarily like but can push him to perform at his best. Imagine if Roy Jones shelved his ego and went to either Roach, Nazim or Steward. Would he have improved or got worse?
What you tend to see is a better career by the guy staying with his original trainer. Especially if he was fortunate to begin his career with a good one. It also comes down to money & a lot of fighters--or managers-- just don't like giving out 10% of the proceeds when the purses get huge. Usually you see the trainers fired after a loss anyway, so recent times see guys switching trainers frequently. What is unheard of is the recent trend of guys like say Rahman, to fire his trainer a few days before a big fight & then import somebody over on short notice. Perhaps its the manager firing the trainer and not the fighter, but it sure seems like terrible timing to be doing something of that magnitude regardless.
Changing trainers can work well, but if a fighter has a Glass Jaw, the best trainer in the World won't be able to prevent that fighter's Chin being absolutely obliterated. See Amir Khan's move to Roach as a prime example.
Wlad switched 2 trainers. Sdunek(who doesn't worse ****), and Tommy Brooks(improved greatly with him but was fired by Universum). To say the least, Wlad would never lost Sanders fight with Brooks. And yes, Jones would have improved greatly with Roach or Steward.
That has yet to be proven! Upon switching to Roach, Kahn has came back from a brutal KO to beat MAB(although a badly faded version) and also win a world title. It seems to me that Kahn is a prime example of a fighter with a glass jaw who has switched trainers and reaped the benefits. I find your choice of him to support your viewpoint rather strange.
4 fights under Roach likely to be 5 after Malinaggi, Khan's chin hasnt been obliterated since under a different trainer, horrible example.