Realistically, when it happens a bunch of people will shut down their rigs b/c of electricity costs, then the difficulty will go down. So although the blocks will be only 25btc it will be easier to get them. Similar to what happened when the $ value of the btc crashed.
Since June, the dollar value of bitcoin has collapsed to 20% of what it was before. The difficulty is currently 200% of what it was in June. This means that buying new rigs right now is likely going to be very unprofitable.
Looking at it from the unsustaible mass media bubble induced prices is silly. In June if difficulty and price had remained constant the annual ROI (@ $0.10 electricity) was about 8000%. Do you know any other investment where you can make 8000% on your capital by plugging in some computers?
It was a bubble and it broke. Mining can be profitable without ever returning to those unsustainable levels.