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.
That fixed point is the price of BTC when you sell them at some date in the future, ideally when it is highest. If you are worried about the price dropping you could sell your BTC as soon as you mine them, or periodically as you choose.
I don't want to get into that sort of speculation with this sheet, that's why I used a set amount. You can play still around with different BTC to USD conversion rates and see how the price will affect your net income.