I've never had this happen to a pony, but it did happen to a Cardcaptor Sakura doll that I've had for years, which had mostly been sitting in storage. It practically fell out at the slightest touch and now she's bald. It made me so sad, but now that I have some rehairing practice I suppose I could try to give her new hair. But yeah, whether it's the hair itself or the glue that holds it in breaking down, giving her new hair is really the best recourse.
As Sawkinator said, it's pretty time consuming but once you get the hang of it, it's fun and relatively easy. Plus, when you rehair a pony you get the chance to add a little creative flair to it, if you want.