“Unless the opposition can unite…”
Not only it is a question whether the opposition to Assad can unite, but under what they would unite. It seems to me that the likeliest replacement to the current Syrian regime is some form of Islamism, which is hardly if at all better than a socialist dictatorship.
I agree with you that a constitutional republic probably would be the stablest form of government to govern Syria, if only it could be established organically. Unfortunately, barring the explosion of the Gospel in that region, the presence of Islamism as an ideology incapable of negotiation or compromise will continue to fuel cycles of violence for generations to come.