Blocktime decreased from 30s to 15s.
Block reward decreased from 5eth to 3eth.
Profitability ratio = (3eth/15s)/(5eth/30s) = 1.2
Theoretically there should be 20% profitability increase.
It would be a bit lower as more people will mine ETH.
Yes, the difficulty will again start to decrease as more new miners will come on-board and old miners add more rigs due to this temporary increase in profitability. After a few months the difficulty will rise enough where profits are back down to what they were just prior to the fork and continue the trend down. People considering getting in to mining now need to factor this in when they run their profitability calculations as what the calculators show today will not be the same in 3 months.