It should only be considered as an indicator.
You can only count in $ per day for your reward if you exchange your coins everyday for $.
Which is most probably not what the majority of people is doing.
What you are earning in terms of btc today can be 14$ at today's exchange rate but be worth 12$ or 200$ tomorrow, but if you did not exchange the coins it is worth the btc amount that's it!
$ value should only be considered once coins are sold and in your bank account.
Stop looking at daily value and think long term or sell your gear, stop mining and do only trading if you want daily $.
I made the choice to mine almost 3 years ago and very happy with it!
You are right.
But if we can pay our hardware like that.
Always same amount in BTC and not change when BTC/USD change, it's will be perfect.
The problem is the mining hardware cost in USD, so if you want to calcul profit you must do in USD.
It's same with immovable, you don't buy building, and calcul your profit in people or anything else
I have a good price for S9 and L3+, i hesitate....
Or D3 maybe.
But i think, S9 is the more reliable at long term.