It used to be easy to calculate the days to break even on a miner and make a decision to buy or not.
False. We only thought our calculations were more accurate. Now we know that calculations are just about guaranteed to be way off.
Maybe you were stupid.
I always understood exactly what I was calculating. When power bills were less than 1%, it was easy to ignore operating expenses, now you cannot do so.
As for difficulty, My 6 month projections have been +/- 10% for the last 2 years.