That's a very silly thing to say. No one calculates ROI on best case scenario and investing by simply expecting something to go up in price is a very stupid investment strategy.

Truthfully, ROI should be calculated by taking the worst case expectation of difficulty increase and price drop to determine what it is worth for you. You can reasonably expect difficulty to increase by 5-10% every 3 days which puts you at a difficulty of over 4000 at the end of the month - now what if priced dropped to 20?

Don't insult people on the basis of a behind the napkin calculation you did.