It doesn't really seem like trainers are despawning at all. They're spawning at a slower rate now but they just keep piling up. I've tried cleansing with fire and swords which helped for a bit but they just come back. Another note, they seem to have too much HP. It takes quite a few hits from a diamond sword to kill them.
Anyway, am I missing something on how tame these trainers? They're breeding like bunearys. At any given point I can see a handful of trainers around me.