The OP's blindside is that he's failing to take into account the fact that bitcoin's price could be dramatically different in the future.  Sure it might be ~$20 today for 1BTC, but are you do you really know if it might or might not be $200+ say two/three weeks from now?  I'm no econ major, but considering the exponential price increases we've observed all but of late, I'd say $200, or maybe even more, is not entirely out of the question a few weeks from now.  If that were to happen, a lot of people'd be dead sorry they gave up mining around this time.

Of course, therein lies the rub.  The price could also tank pretty drastically in the future.  No one really knows.