The "moderate Syrian rebel" has been an endangered species since at least 2013, if not before.
Now, with the latest success of Assad's army, moderate rebels are destined to become as rare as unicorns.

Syria's army and allied militia fighters seized the only road into the rebel-held part of Aleppo on Sunday, tightening a siege around opposition areas of the northern city, which President Bashar al-Assad has pledged to recapture.
Aleppo has been a major battlefield of Syria's civil war since rebels swept into it in the summer of 2012, and an opposition defeat there would mark their biggest setback in five years of conflict...
"We are now besieged and you don't have any tunnel or any strategic stockpile that lasts for long.. only for two or three months to feed 300,000 people," the commander, who asked not to be identified, told Reuters.
He predicted a lengthy blockade before an attack on the main rebel districts. "After two or three months you will start getting hungry and can no longer resist, and then they will storm the city," he said.

The siege of Aleppo is already hurting, and the UN estimates that people will be starving within the month.

An Assad victory at Aleppo won't end the rebellion, not even close, but it will do two things.

1) it will effectively end any pretence of the Syrian rebels being anything but jihadists, because almost all other rebel-held areas will be under the complete control of jihadists.

2) it will effectively end any hopes of overthrowing the Assad government short of a U.S. invasion, and the international community knows it.

As that battle raged, the US was drafting a deal with Russia that would create a joint operations centre to coordinate attacks on al-Nusra and Islamic State.
The move has created despair among the ranks of the Syrian opposition, which insists that a pact between Moscow and Washington will entrench the Syrian leader, whom Russia and Iran have saved from defeat over the past 12 months. Adding to the alarm of the now diminished rebel ranks is a detente, also signed during the week, between Moscow and Ankara, after a seven-month standoff, as well as the Turkish prime minister’s remarks that Ankara was interested in peace with Damascus.
“This all means that Assad is no longer at risk,” said a senior official in the western-backed Syrian opposition. “This means that he has won.”

Well, Assad hasn't so much won as he has managed to avoid losing.
From the start, the civil war in Syria was a proxy war. The rebels were directly armed by the CIA, Turkey, and the Gulf states.
On the other side, Assad was propped up by Iran, and lately by Russia.
Neocons and liberal war hawks have raged against Russian intervention (while rationalizing our own intervention), but Russia isn't even the biggest player here.

Even in the unfathomable event that Russia were to abandon Syria, direct American military action would cause Iran and Hezbollah, the Assad government’s closest allies, to intensify their support. This would strengthen hawks in Iran and dim prospects for further improvement in United States-Iran relations.
Perhaps the interventionists believe that American military action would force Mr. Assad to the peace table. That prospect is equally implausible. There is no conceivable bargain that the Syrian president could strike with his adversaries, many of whom are hard-line Islamists. He and his colleagues would rather go down fighting than hand Syria to Sunni jihadists. The same goes for Iran and Hezbollah.

Not only would direct U.S. intervention derail Obama nuclear deal with Iran, it might lead to all-out war with Iran.
Are liberal war hawks prepared to take that step?

As for the remaining rebels, it appears that ISIS won't be around very much longer.
The Kurdish-led SAC conquest of Manbij is an enormous defeat for ISIS.
Combined with Baghdad running ISIS out of Iraq's Anbar province this year, the jihadist group simply doesn't have the strength to do more than suicide bombings anymore.

So if the moderate rebels are almost gone, and ISIS is being defeated, who is left to oppose Assad?

“In Syria, as [IS] is losing territory in the east, its terrorist rival — Jabhat al-Nusra — is gaining ground in the west, putting down roots in Idlib province along the Turkish border,” Brett McGurk, the US special presidential envoy for the Global Coalition to Counter IS, said June 28 in written testimony to the Senate Foreign Relations Committee.
“With direct ties to Ayman al-Zawhiri, Osama bin Laden’s successor, Nusra is now al-Qaeda's largest formal affiliate in history,” McGurk said. “This is a serious concern, and where we see Nusra planning external attacks, we will not hesitate to act.”

Unlike ISIS, Jabhat al-Nusra managed to withstand the Russian/Iranian/Assad assault without giving up much ground, and has lately managed to hit back.
Al-Nusra has repeatedly defeated U.S. attempts at creating a moderate rebel force in Syria, including just last week.

Syria's al Qaeda-linked Nusra Front has abducted the commander of the Western-backed Jaish al-Tahrir brigade, along with several of his aides and scores of fighters in coordinated raids in northern Syria, Jaish al Tahrir said on Sunday.
Jaish al Tahrir was set up in February as part of an effort to forge unity among moderate rebels in the Free Syrian Army (FSA) alliance at a time when a major Islamic State advance threatened their main stronghold near the Turkish border...
Nusra Front fighters stormed several locations in coordinated raids and set up checkpoints to arrest around 40 fighters, the group said.
The Nusra Front has previously targeted rebel groups supported by the West, leading to the dissolution of the Syria Revolutionaries Front and the Hazzm movement last year.

For those of you who follow the Syrian civil war, this might sound suspiciously familiar. You aren't imagining it.

Al-Qaida’s affiliate in Syria has killed five members of a rebel group trained by the US and wounded 18 others, a day after kidnapping seven members of the same force, in the latest blow to American strategy in Syria.
With the kidnappings on Thursday and the fighting on Friday, the number of US-trained fighters put out of action by Nusra’s campaign has risen to 30.

So the next time the US-trained rebels encountered al-Nusra they simply handed over their weapons to them.

Abu Khattab al-Maqdisi, who also purports to be a Jabhat al-Nusra member, added that Division 30's commander, Anas Ibrahim Obaid,had explained to Jabhat al-Nusra's leaders that he had tricked the coalition because he needed weapons.

Then just last March, al-Nusra overran a base for U.S.-backed rebels and stole all their weapons again.