We forget sometimes that this is a business - not a sport. MS outcoached Sony at the start of the generation - remember how smug Sony was when it unveiled the PS3. Things will shift back Sony's way in time. These things are cyclical. Games that were only on Xbox are coming to PS3 eventually too.
There's too much money available - and dev costs are too high - for anybody not 1st or 2nd party to go exclusive for too long. And the number of units being sold on these big franchises will put the price tag for buying exclusives in the major franchises WAY too high for it to make sense.
A minor, temporary blip on the radar in the grand scheme of things.