You can't because you won't know what the difficulty will do without knowing how BTC/USD is doing. The two are correlated, we just don't know how. It's like asking which came first, the chicken or the egg. Does the difficulty go up because BTC/USD is doing well, or is BTC/USD doing well because the difficulty is going up?
You can't assume that the difficulty is going to just keep rising without BTC/USD rising with it, or people wouldn't keep plugging in their miners, as mining would no longer be profitable.
Yes BTC/USD and network hashrate are very likely correlated.
But here's the thing, there is a massive lag between the numbers (caused by Bitmain mostly), so the enormous amount hashing power coming online now and in the next few months is a result of BTC/USD increase over the last 6 months.
It could very well flatten out again once this all settles but what the spreadsheet shows is that you would need it to flatten out very, very, very, soon, like in the next month or two, even quicker than the most optimistic scenario presented in the spreadsheet, for an S9 to pay itself back and become profitable in 2018. And be serious, look at this chart:
https://data.bitcoinity.org/bitcoin/hashrate/5y?c=m&g=15&t=aYou really think that is realistic?
You could have made the same spreadsheet and made the same predictions a year ago and said: "Look, the difficulty is predicted to keep rising, it would be better to just buy BTC and not a miner because a couple months from now, your miners won't be making anything". That wouldn't have worked then for the same reason it doesn't work now, because you're not assuming anything about BTC/USD!
You are spot on.
Up until very recently mining has never been profitable for most of the Western world, only where power is dead-cheap was it profitable but on average most of us were just breaking even or below the profit line.
In the Western World it was always more of a hobby thing, you could always make more money investing directly into your favorite crypto.
It's only because BTC value has increased so much over the last 6 months that all of the sudden an opportunity has presented itself for everyone to make money.
But network hashrate is quickly catching up again, and BTC value is now staggering so it will probably even overshoot before bringing things back to balance. (meaning a period of profit for nobody at worst)