The situation is grim for both sides, really:
With the victory in Aleppo the Assad government forces (over 250,000 full and part time fighters) control nearly half the country and about 70 percent of the population. Most government forces are for local defense. The rebels have about as many fighters as the Assads but they are not as well armed, trained or united. Only the FSA (representing about a quarter of the rebel fighters) is acceptable to all the nations supporting the rebellion (Turkey, the West and the Gulf Arabs). The Kurd coalition (the SDF) has about 20 percent of the rebels but is not recognized by the Turks because it contains a lot of Kurdish separatists. The Islamic radical groups (often referred to as the Islamic Front coalition) account for a third of the rebels and are still supported by some Arab nations and used to be (and might still be) supported by Turkey. The remaining 21 percent of the rebel fighters are the Islamic radicals who are out to conquer the world but split into several factions that are at war with each other. These groups have little outside support. The worst of them is ISIL, followed by the local al Qaeda franchise (formerly al Nusra) which is still cozy with al Qaeda. It was the Islamic radicals who, more than anyone else, prevented the rebels from uniting and overthrowing the Assads by 2014 or 2015.
If the side aiding the rebels would go all in to defeat Assad, it would help the non-jihadis again dominate the rebellion. The Obama administration has asked rebels to either fight and die to destroy ISIL--which aids Assad as a side effect; or to fight Assad in order to pressure him into negotiations.
But note this:
At least half the rebel fighters are more anti-Assad rebel that devoted believer in whatever other political or religious agendas the various rebel factions have. These rebels have switched from one rebel faction to another over the last few years, depending on which coalition seemed most likely to possess a winning strategy. From the beginning the two main coalitions were non-religious (seeking a democratic government) or religious (seeking an Islamic dictatorship or a democracy that recognized Islamic law). Since the Russian intervention in late 2015 and the Turkish invasion a year later, many rebels have switched back to non-religious coalitions like the FSA or SDF. [emphasis added]
Do read it all.
But is the rebellion over? I don't see why Assad's recovery must be the last change of fortune.
If we help the non-jihadi rebels, it will make them the strong horse at the expense of the jihadis. That will gain recruits for them at the expense of the jihadis. I've been arguing this for some time as opponents of arming Syrian rebels have instead searched for perfect rebels to back.
Assad has few ground forces suitable for offensive action or even the ability to move around the country in reaction to threats. So reconquering lost territory and people is not a given.
If the foreign states backing the rebels increase support for the rebels, it will add to the financial burden on Russia and Iran which provide that offensive power.
If we add to the financial pressure on Russia and Iran, we make it tougher for Russia and Iran to afford to pay for that offensive units of Iranian, Russian, and Hezbollah assault troops plus the Shia foreign legion that Iran has organized.
And Hezbollah has bled a lot already and doesn't seem fully on board their cannon fodder role any more.
Maybe if we had information war operation where Iran recruits Shias we could reduce that capability. We should be praising Iran and the dead for sacrificing Arabs and other non-Persians for the greater glory of Iran. Make it obvious that Iran is willing to fight to the last Arab Shia for their imperial glory.
We also need to help the rebels build their own mobile force that can both react to Assad offensives and, more importantly, hit Assad weak points to attack them.
We also need to work on keeping the Kurds from cutting a deal with Assad that takes them off the board. We can't count on Kurds to march on Damascus with other rebels, but we might at least keep them from helping Assad once the fight against ISIL is largely won.
If we can continue to build up the Arab forces now helping the Kurds fight ISIL, we may overcome the loss of the ISIL forces in fighting Assad.
Remember, while Assad may control territory with 70% of Syria's population (I assume that means 70% of what is left inside Syria), Assad still has a smaller recruiting base for reliable troops. And that base has suffered enormous casualties. I don't assume they will continue to fight and die for Assad if the war drags on without that light at the end of the tunnel which Assad, Russia, Iran--and now Turkey--want all Syrians to believe is the end of the civil war in sight.
If Assad wins, it will be another 30 years before resistance to Assad from the Sunnis can rebuild.
And in the meantime, Assad will take revenge on America. We won't get any credit for refusing to "further militarize" the struggle, as our government famously said early in the civil war, by deciding not to go all-in to defeat Assad.
No, Assad will seek--again--to kill Americans and undermine our position in the region with their friends the Russians, the Iranians, Hezbollah, and sadly Turkey perhaps.
We struck a king (Assad). We should kill him or expect revenge.
UPDATE: Another overview of the situation.
This claims more Assad forces and far fewer rebel forces--although the latter could be a problem of not counting rebel part-timers who would be in the local defense forces that predominate (on both sides).
And the article clarifies that the population control is of those who remain in Syria.