I have trouble believing vast majority of miners are not selling for fiat immediately, because if a miner is not converting to fiat, then he's just paying facility/electricity/equipment cost out of pocket, then he's essentially buying BTC with fiat. He could have just bought BTC on an exchange if he wanted to buy BTC with fiat.
Vast majority are (IMO) selling, because the vast majority are the corporate mines that aren't interested in BTC as a technology (holding). They're interested in making millions of fiat dollars. As for the indies, I keep most every coin I mine and pay my expenses out of pocket. Why you may ask? Probably because every coin I mine has a cost basis of <$200. Can't get those on the exchange.