It is an estimation - certainly a good back of the envelop calculation, but lot of assumptions. It is upto you how you take those assumptions.
Its not all about roi, think about there will only ever be 21m btc,
I think as it like this,
Lets say there will ever be 21m kg of gold, now you can mine 1kg of gold every day you wouldn,t care about the fiat price against it becourse you know in the end everybody wants your kg of gold.