Steve Saideman has a good post today about the reasons the violence in Syria will go on. I agree with everything in it. I would merely pile on another reason for pessimism: the narrow base of the Syrian regime, with Alawites dominating the armed forces and security services.
Steve argues that Bashar al-Assad cannot make a credible commitment not to use violence. I think this is true, but I also think commitment problem cuts both ways here. I argued in a paper [gated; sorry; working on ungated version] in 2010 in Comparative Politics that narrow regimes in the Middle East, Syria included, have been able to limit military defection, because the in-group basically gets locked into loyalty. Since being an Alawite (or a Christian) is a relatively reliable predictor of one's support for the regime, Alawites are likely to suffer serious political marginalization if ousted from power. I (and others) have seen this continue in the Arab Spring. I suspect this is even more true given a divided opposition, in which competing factions may have incentives to appear more thoroughly anti-ancien-regime.
Further, I suspect that the narrowness of the elite means that an offer to Assad and his inner circle for exile and immunity is unlikely to be effective. It's not only they who need the sort of a credible commitment that exile affords. It's about 10-20% of the population, including elite units of the Syrian armed forces. They can act as a veto player over any arrangement where they get cut out.
It's for similar reasons that James Fearon finds that civil wars that arise out of commitment problems last longer than others. And Lindsay Heger and Idean Salehyan find, in addition, that the narrower the base of an autocratic regime, the more violent a response they pursue against armed opposition.
The argument above is really about why the regime will be difficult to oust, short of military defeat or an extreme of exhaustion. What about the ceasefire plan, though? I think that it's unlikely to work by extension. Members of the Syrian opposition intend, as makes sense, to use the ceasefire to take to the streets again. If violence is actually off the agenda, it's a clear win for the side with the military disadvantage. It is hard for me to see the Syrian regime giving up its advantage here.