A proper ROI calculation is anything but simple. 

If you had made acceptable assumptions in July/August looking forward you would have seen Hashfast, Cointerra, BFL, Terrahash, Avalon chips GB's, ASICMiner Gen 2, KnC, etc in the pipeline.  The outlook was horrid!  Fast forward to today and how many of those expected offerings have come to fruition?  In September people were running around waving the genesis block calculator results which predicted a difficulty of 4 Billion coming into January.

It's a concern yes, but a lot of people running around making these claims really have nothing to back them up.