In other words, every unit being shipped [late] by butterfly labs right now will never see a profit.
Paper weights!
The reason most people are on the low side is that they're not taking into account the massive recent spike in Bitcoin Price [which will keep surging into Christmas and New Year] and also the global attention [and legitimacy] brought to Bitcoin due to the closure of Silk Road.
We probably now have a few million new entrants which nobody is aware of, and that number will sky rocket next year with the Bitcoin ETF.
So both of these factors [and soon the Bitcoin ETF] will greatly accelerate the difficulty rate as millions more people buy ASICS, which will consistently put a massive downward pressure on Scrypt coins which means Litecoin et al are doomed.
But I digress...
can difficulty of BTC decrease
Of course. If people choose to mine something else or just turn off their ASICS then the difficulty will go down.
maybe that will happen sometime in near future, when ppl stop buying their miners and quit mining cuz of ROI