Fact number 1: A united and peaceful Syria ruled by Assad is simply not possible anymore.It has been like that for some time.The status quo ante cannot be restored. Iran and Hezbollah realize this more than anyone else.
Fact number 2: The Assad regime is incapable of adapting to a powersharing arrangement as contemplated by the Geneva principles. The regime is brittle and fragile as it is brutal and ruthless. It can break but cannot bend. Assad knows it and Iran knows it.
Fact number 3: A free and democratic Syria would be a strategic disaster for Tehran. If given a choice, the Syrian people would be certain to sever their country's geopolitical alliance with the Islamic Republic and stop providing a geographic corridor to Iran's military arm in Lebanon..
Fact number 4: Iran's second best alternative to the irretrievable status quo ante is simply a protracted war. This is now Iran's victory strategy. A bloody and chaotic Syrian theater will still be usable by Iran and Hezbollah more flexibly and efficiently than their western enemies. Remember the civil war in Lebanon?
Fact number 5: A protracted war in Syria will help terrorism flourish even more. Both the kind manipulated used by the regime to blackmail the west and the "authentic" strain that festers and spreads in open wounds, like opportunistic parasites.
Conclusion: If Iran's militant ideology and hegemonic ambitions and radical "Islamic" terrorism are the two strategic threats that need to be overcome, then the policy towards Syria should aim at bringing to a quick end both the devastating war and Assad's rule. Humanitarian considerations aside, any policy that is based on the premise that a protracted conflict in Syria is costless is misguided and dangerous. It is exactly what Iran wants and it will help the scourge of terrorism to thrive.