It appears to me that the administration is going down a bad path. Assad is scum but he keeps the real scum in check and the LAST thing we need in the ME is another power vacuum. I'm not convinced Assad is responsible for the sarin attacks and I'm not sure I care even if he is. We should be supporting both him and the Russians - moderately. In order to maintain some level of influence with both and improve relations with Putin. Mostly we should avoid expending much blood or treasure in Syria. We need to stop naïve 'nation building' in Islamic countries.
I do believe that Iran and Russia would have him stay in power....especially Iran....Russia is a bit flexible on that note....Putin doesn't want war on his border and is unlikely to want to give up his ports to the Mediterranean. At most he;ll have to share power and I don''t think Assad would agree to that. so to me his options are minimal at best now.