> Arab democracies are unstable because they will elect Islamists.
why does that imply instability?
Because every Islamic theocracy to date has been profoundly destabilising for its neighbours and the world, and always ends up imprisoning and immiserating its own people. No one wants more Irans or Afghanistans. (Or Saudi Arabias, though that's not said out loud as often.)
Fundamentalists tend not to be pragmatists.