There's no guarantees that these miners will be fine in the long run. That's why they'd be selling their BTC for fiat trying to expand their hashrate asap and getting a bigger share of the pie and maximize their profit potential. The mining companies who are simply holding their BTC as you say will go the way of the dodo soon.
Miners are industry insiders at the top of the pyramid. I'm sure they are pretty confident about BTC's future.
BitFury founder and CEO Valery Vavilov indicated that the new funding will allow the company to complete production of its 28nm ASIC chip without selling the reserve bitcoins it has mined from its three industrial-scale data centres.
Vavilov told CoinDesk that it decided not to tap its bitcoin reserves as it remains bullish on the long-term value of bitcoin.
"We believe in the long-term perspective [the price of bitcoin] will grow and we decided to not to sell [our bitcoin] at such a low price," Vavilov added.
Lol, a company whose worth is pegged to Bitcoin price is not going to announce "we're dumping these things as fast as we mine them, because don't believe in the future of Bitcoin."