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.
You are telling me you were able to accurately calculate the difficulty growth rate even with a huge amount of unknown variables like value of btc, hardware sold, hardware being used, new asic manufacturers, private farms, etc..
Can I borrow your magic crystal ball? Or did it stop working all of the sudden?
Seriously though, why can you not make reasonable estimations now if you could a few months ago when much less was known about the effect of asics?