If Honda threatens the nuclear button, they have a lot of power. Should they leave, their team (McLaren) would automatically be prevented from scoring a single point in the championship and would be competing for honour and Bernie money only. That would be a severe embarrassment to the FIA even if it doesn't like McLaren, and makes it more likely that it will have Force India (the loudest relatively stable team) in the Strategy Group again in 2016. Plus it will scare Red Bull as it does not produce its own engines and Renault have made noises about leaving (further meaning that Lotus getting into the Strategy Group wouldn't solve the FIA's troubles from that direction). Williams wouldn't exactly be happy either...