Why is it that all of these predictions assume a fixed price per BTC...?
How would your graph look if at 150,000,000 difficulty that BTC is trading at $400/coin?
It's not fixed. You can change the BTC price to whatever you want. It's just defaulted to the current price.
Yes but your speculation on loss of profitability stems entirely from using that fixed price point.
There is literally no way to even speculate on future pricing of BTC, with any kind of claim to accuracy. It could be $400, or $4000 or $400,000,000, or it could be $1. You can be bearish, or bullish, and you really can't back it up with anything beyond that.
You can certainly speculate on hashing power, based on current available data, pricing, orders-to-date, etc., within a reasonable set of bounds.
To create a spreadsheet predicting difficulty is entertainment, to predict price, madness.