The game theorists assume at the outset that everyone involved's primary financial interest is in a valid working Bitcoin network. However, banks and Western Union would not desire competitors for international currency transfer. Governments may not want currencies they can't control. The Russian mafia may want to exploit it for short-term gain or launder money. And now that there's a single pool with >51% of the CPU power, the difficulty of a hostile takeover just went way, way down.
Before, a group would have to set up a massive amount of computing power in order to take it over, but now, taking over or disrupting Bitcoin is within the reach of non-technical entities like criminal organizations and the CIA. E.g., all the CIA has to do is threaten a few people and say "Insert this code so we can freeze the transactions of $VILLAIN_OF_THE_MONTH at any time." There's many more possibilities now.