If Assad falls, the worry is that ISIL wins Syria. I don't believe that is true, although the war will continue. Don't let worry about ISIL interfere with the defeat of Assad. Saving him or his regime won't "responsibly end" this war.
Strategypage notes that Iran and Russia are looking to ease Assad out of power. If that happens, the Assad regime will continue--just without Assad. A shiny "new" face won't change the situation.
Iran and Russia still have reasons to support a post-Assad regime, remember. Iran wants land access to Hezbollah to have a direct front with Israel; and Russia wants a military presence in the eastern Mediterranean, which the port of Tartus could provide them.
And Iran is increasing its control over the war, as I've noted before. So it doesn't look like the fate of Assad and his inner circle have made the fate of Syria less important to Iran.
Note this map of control:
Assad is largely driven back to the region that I said over three years ago that he had to retreat to. As his situation deteriorated, I thought he could rebuild in that core and then retake his territory in time.
Now it is just an attempt to survive. Yet retreating fully to that core is probably too late.
Now, Assad's forces have suffered too many casualties to hope even his most loyal supporters will die for Assad in a smaller realm (and the Stratfor piece says the fall of Taqba air base last summer was as significant as I thought it was).
So what if the Assad regime collapses? Does ISIL control it all?
No. ISIL will still control large parts of northern and eastern Syria. They will likely exploit the collapse of Assad's forces by grabbing pockets in the east and center.
But large forces of rebels (including non-ISIL jihadis, unfortunately) will hold the regions they are in now--and take over some Assad areas. The biggest prize would be Damascus.
I expect the 80,000-strong Iranian-supported Syrian militias will still hold their own Alawite regions, buttressed by armed forces units that retreat to Alawite core regions near the coast.
Hezbollah will still fight west of Damascus to protect their own areas in Lebanon.
Kurds will still hold terrain in the north.
Southern Front rebels that we back will still hold the Israeli and Jordanian border regions in the southwest. With some effort, maybe some of Assad's regulars would defect to the Southern Front rather than run the gauntlet to reach their homes farther north.
We'll still be bombing ISIL in Syria.
And we are doing something about arming non-jihadi rebels, since those contractors who died in Bulgaria were allegedly testing weapons destined for them.
And Turkey's large and capable armed forces will remain looming in the north--although domestic politics may freeze Turkish potential for action as the system digests President Erdogan's electoral setbacks.
Israel also looms over the southwest as a rebel rear area and potential source of air support.
Jordan, too, holds the ring in the south where we have troops to support the Southern Front rebels.
So ISIL won't have much more than they have in Syria, which is mostly in the eastern half of the country.
And forces aligned against ISIL will still control large parts of core Syria in the west.
I've long held that the war in Syria has to be a two-part fight. First defeat Assad and then defeat the jihadis. But we dithered.
It is really a three-part fight now that the Syria Civil War has (since January and then again in June 2014) spilled into Iraq, since after Assad we have to defeat the ISIL portion of the jihadis which threatens Iraq before addressing the rest of jihadis in Syria.
So let's not save Assad or an Assad-less Assad regime (and save Iranian and Russian interests). Prepare for the collapse of the Assad regime and prepare to win in the Balkanized territory of Syria by defeating ISIL while building up non-jihadi forces in Syria (including getting elements of Assad's military to defect to rebels we back) to defeat the rest of the jihadis after Assad is defeated and after ISIL is defeated.
UPDATE: This author doesn't think Iran's hopes to hold western Syria to maintain a front against Israel via Hezbollah will work.
It does look bad for Assad absent a dramatic development--direct Russian and Iranian troop support. Assad could be in such a bad situation that even a decision by President Obama to pump money into Iran and become Assad's anti-ISIL air force can't save Assad.