Are you assuming that they will continue to sell mining equipement for same number of BTC ?
Event if BTC price is 10 or event 100 times more expensive ?
Like a jalapeno for 27400 $
If not, I don't understand how you expect they will regain theyr money ?
(Sorry for my english)
Consider such a case:
Equipment maker sell a mining rig for $100, make $50 profit. The miner who bought the equipment will mine $200 worth of coins, sell part of the coins, like $20. And equipment maker buy back those coins with $20, they will be able to make $30 while still digesting the added coin supply, thus keep exchange rate stable
However, if the miner want to sell all the coins mined (pure fiat profit driven), then they will dump $200 worth of coins into the market. The equipment maker can not digest such large amount of supply with their $50 profit. Price will go down, equipment maker can not sell a profitable mining rig, they lower the mining rig price to their breakeven point, and miners can not mine enough coin to ROI, they start to hold coins and coin supply will decrease
So it depends on miner's action, if they are fiat profit driven, they will drive the price down, if they only cash out a little bit every year, then the whole system will grow fast and stable