Mining $2 per BTC @ 1.7M difficulty is like scouring the roadside for aluminum cans to return, except imagine every day the cans are worth a little less.
Difficulty hasn't been 1.7M for some time now.
Also when difficulty was 1.7M price wasn't $2 so I am not sure what you are trying to say.
Then again I don't think you are sure what you are trying to say.
Also none of this has anything to do with your claim that mining @ $2 is impossible. If difficulty fell back to 200K would it still be impossible? If BTC was $20 USD but difficulty was 40 million how profitable would you be.
Simply claiming $20 BTC is better than $2 BTC is meaningless. Higher BTC is better for speculators. It has nothing to do w/ economics of mining.