This wouldn't be fair to people who played conservatively and saved their dust/candies instead of dumping it on anything just to sit in a gym.
I wouldn't want this to happen even though I have 2 maxed Lapras which has been mostly on the sidelines.
I assume if they made it 1 Pokemon max per year or something, maybe that would be ok so dust wasters won't get everything back, but my guess is any data you spent on dust is long gone and they won't be able to retrieve it.