But you can't base the full value of the miner on today's exchange rate alone, which is why these predictions don't make sense. You assume the difficulty will continue to rise, yet not the price of bitcoin. This prediction is made in an unrealistic vacuum that isn't applicable to real life, at least not the way that I'm understanding it.\
Yes you can, if you convert that $3000 into BTC TODAY and you hold from that point on in both scenarios you absolutely can. BTC/USD exchange rate is not relevant anymore because we're comparing BTC-BTC at that point.
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 in terms of USD, which is what people use!
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!