To win a war, to really win, you must break the will of your enemy. What would break them more than to know that assad, with putin’s help, could use chemical weapons with impunity? He did the calculations and decided he benefited. Meanwhile, his champion and patron, tell the lies that many swallow whole cloth, their reflex anti-Americanism making them vulnerable to the lies of a KGB thug.
I don’t specifically believe Putin, as all politicians lie. It is simply a matter of figuring out which lie, when.
The will of which enemy is he trying to break? ISIS? Qatar? Saudi Arabia? Saudi Arabia has made it clear by bombing of Yemen they are not terribly concerned with civilian lives. As long as the Gulf states finance a rebellion, there will be a rebellion.
You can also win a war simply by breaking the enemy physically and Assad and the Russians have more than enough firepower to do that.
I suspect in order to really secure Syria long-term, a few Saudi agents and arms dealers will have to be dealt with discretely. (or not discretely, depending on the message needing to be sent)