- include estimates for difficulty for next 5 rounds in your projections
- make different estimations based on varying BTC price
Doing this you can still get a pretty good idea when you will pay off your mining equipment.
And yes, you are correct, there are no guarantees, it is risky and not easy.
But the decision is based on your math in the end. (and risk tolerance once you know your numbers)
Yes, doing the maths is a good idea. It will give you a more clear vision of what could be. Im doing it.
The problem is that at the end of the day all your maths could account to nothing. The bitcoin price exchange could skyrocket again and give you record profits as the new miners take a while to set up new rigs. Or the bitcoin price exchange could collapse and you would eat all the hardware.
You can do all the maths you want to get a clearer vision of the "what if", but no math is going to tell you what is the right decssion.