Under either scenario, Islamists would remain a significant factor in the political game, as they deserve to be in a democracy; their influence would undercut the arguments of religious zealots who claim that democracy is a sham.
But this need for cooperation across the religious-secular divide wouldn’t be met under any presidential scenario. Once the military assures the election of an acceptable president, Islamists would probably walk out of the legislature. Even if they remained, they would predictably use their parliamentary platform to denounce Mr. Morsi’s replacement as illegitimate. If they tried to block legislation, the new president could push through his program without their support by greasing the wheels with political patronage and outright corruption. If that failed, he could make aggressive use of his executive powers — generating cycles of alienation that would, over time, undermine the very ideal of democracy.
Parliamentary government is no cure-all, but a good design can remedy the most serious pathologies. Some systems, like the Italian, require a government to fall whenever a majority of representatives votes “no confidence” — leading to notorious episodes of instability. Others, like the German system, keep the old government in power until the new majority can actually agree on a replacement. That is by far the better approach for Egypt. While momentary majorities may say no to government initiatives, they should show that they have the sustained support of Parliament as a body before they can establish themselves in power.