Had Constantinople fallen in 718 (a ridiculously unlikely event by the way) then there is nothing really to stop the Arabs advancing to conquer all Europe. Of course, they cannot carry on indefinitely, but to see all of the old Roman Empire under Muslim domination is not too unreasonable. Eventually, cracks in the Caliphate would have slowed their advance, personally I cannot see the Arabs advancing too far into Britain or Germany, if these areas were too cold and wet for the Italian Romans, I can certainly not see the Arabs conquering them.