Letting Assad stay in power is literally the worst thing that could happen - even turning the whole country into a radioactive desert would be preferrable. If Assad stays, we send a message to all other despots around the globe that slaughtering your population is a good way to stay in power. Such a decision would in the long run result in milllions of deaths.
We've sent that message already. We can not unsend it. All we can do is modify it to "It's okay to slaughter your population to put down rebellion
so long as you don't use gas - at this point, we have no standing to say it's not okay
at all, because we've spent so long saying we don't care if he does.
And deposing him will CONTINUE to send a message that the US will take unilateral action to curb stomp whoever they don't like, against the wishes of the international community, whenever they want. Which, is far as I'm concerned, is a message we don't want to keep sending at all.
My suggestion?
Let France handle it. Renounce our position as world police, stop trying to argue we are responsible for every terrible thing that happens in the world (and as a result ending up responsible for every terrible thing that happens in the world, because let's be honest we REALLY SUCK AT INTERVENING). We keep arguing this is an international tragedy - well, that's the perfect excuse to
let someone else handle it, because the international community is large an frankly, we have lost all credibility for being able to successfully lead a war again a foreign nation, and have a pretty bad reputation for trying anyway and making things worse. It is obvious, pointedly obvious, that "making things worse" is the most likely outcome of our involvement at any level, because we've already fucked up all our chances of making things better.
We can't undo our mistakes, but we can let someone else make the mistakes from here on out.