Ok, let's try again. This machine costs 3.5 BTC. It returns 2.5 BTC in a year. You lose 1 BTC.

I can just buy 3.5 BTC and then not buy the miner. In a year I still have 3.5 BTC. No matter what the cost of BTC is I have more $$ than if I bought the miner.

Yes, BTC price increase will affect your fiat ROI. But look above. If you can't ROI in BTC you would have done better to hold the coin. If BTC goes up, you have more fiat if you hold the coin. If BTC goes down you lose less money if you hold the coin. Mining at a BTC loss is willingly increasing the risk to your fiat while simultaneously diminishing the returns of your fiat. You make less for higher risk.

Would you buy 4 gold bars to trade for digging equipment designed to dig up 3 gold bars and then hope for the price of gold to go up? No you wouldn't, you would just hold the 4 gold bars. Why should BTC be any different? Why would you pay more BTC than a machine returns?

This is why you run ROI in BTC. If you can't recoup BTC, hold the coin. If you can recoup coin plus more, decide if the 'plus more' is worth the risk (will the machine and variables hold up).