I've always heard that they are blemish, but are a sign that the horse has had hard use. My Mustang has them, and it's my fault because I trotted him on a dirt road on ONE ride. One ride, and he's blemished for life.
They may go down if the horse isn't used for a while, but I've heard they are permanent because once the pocket that fills with fluid stretches out like that, it will always be prone to doing that. I haven't heard about the cortisone injections, so I don't know if they help.
The good news is, they are generally just a blemish. My Mustang is as sound as can be. But I hate it that he has them, because when I bought him he didn't have them. But I learned from that experience.
Anyway, maybe others can shed some more insight. But I think it's most likely permanent. I have even heard that they can be drained, but that doesn't do much good because the pocket just fills up with fluid again.
Did the vet think the cortisone would shrink it? My vet has never mentioned anything like that. But maybe I didn't ask and just assumed they were hopeless.