If we had established the no-fly zone earlier, Russia would be in the position of upsetting the status quo and provoking a US-Russian clash if they started bombing anti-Assad rebels.
Now we would provoke a possible clash by establishing and enforcing a zone in the face of Russian air strikes.
We'd have a bigger problem in that while we could no doubt smash the Russian air expeditionary force and destroy their ground-based air defenses in Syria, Russia would retain the ability to escalate in a region where they have conventional military superiority--against the Baltic NATO states.
Russia would totally trade the loss of their expeditionary force in Syria for a pretext to go after the Baltic states and hopefully break the NATO guarantee by negotiating the end of a Russian-NATO confrontation with Russian troops still holding territory in Estonia, Latvia, or possibly even Lithuania.
Heck, even if we could guarantee no US-Russian clash in Europe, we'd still give the Russians a chance to dent our military reputation by shooting down an F-22 over Syria.
So risking a direct US-Russian conflict in Syria is not in our interest.
That doesn't mean we do nothing, and let Russia have a shot at preserving Assad in a Core or Rump Syria based on the Alawite homeland in the northwest to Russia and Iran a secure base there to influence events in the eastern Mediterranean and Lebanon, allow Iran and Russia to push us out of Iraq, and solidify a pro-Russian Shia arc across the northern Middle East (which would encourage Iran to make further efforts in Bahrain and Yemen--and within Saudi Arabia itself).
We need to strengthen the national Lebanese government and weaken Hezbollah in Lebanon, figure out how to use the existing raw material of the Iraqi ground forces to make progress fighting ISIL in Iraq, promote revolution in Iran by supporting dissidents, and send anti-aircraft weapons to rebels in Syria to build an indigenous de facto no-fly zone that local forces will attempt to enforce by contesting the airspace over northern (and southern) Syria.
Smile at the Russians, offer to talk at any opportunity, push humanitarian aid to Syria (even though much will go to jihadis and Assad supporters), and send weapons to help the rebels hold their ground and contest the air space above them.
I'd focus more on automatic weapons in the 23mm range with only small numbers of older models of hand-held anti-aircraft weapons.
And I'd try to get mortars and rockets and recoilless rifles that rebels can use to bombard airfields to damage aircraft that way.
We have to do this even at the risk of weapons leaking to jihadis. Jihadis have plenty of weapons even with our refusal to risk the bad publicity of enemies using our weapons. Yes, the conflict was militarized without our help.
I'll recall our experience in the early parts of the Korean War (from This Kind of War, p. 105):
Second to the Soviets, the American Army became the principal supplier to the [North Koren army] of guns and ammunition.
Our troops abandoned weapons. So are we to expect that non-American forces will never lose weapons to enemies? This is a cost of doing business and not an excuse to not conduct the ugly business of trying to defeat foes.
We should try to win. Or at least try to inflict a defeat our foes.
UPDATE: Oh, and given that it will be difficult to safely provide air support for rebels in the western part of Syria because of Russia's planes and air defenses, why not use MLRS-launched ATACM long-range precision missiles placed in Turkey and Jordan? They have a range of 180 miles.
These long range missiles would help support rebels without risking air-to-air combat.
And if rebels start pushing away from the borders? Well, by then the Russians will be evacuating their forces and the skies will be cleared for our air power.