If Sunni Islamic extremists end up on top locally after the Syrian government falls it will still take them a while to get enough in order to pose a threat to the US and they have a long list of enemies. There will be some cost to the US, and there is high risk but it is slim, but with the likelihood of a very advantageous outcome for the US.
The most likely scenario, should Assad be defeated, is a fundamental Sunni led, Islamic regime taking power with ties to Al Quida that is far more extreme than Assad. This would be a disaster fot the US and probably lead to war with Israel, which would then involve Iran, and have the potential to pull in Russia and the US on opposing sides.
Assad is a far, far more stable player, even if he is not friendly to the US. Despite this, the Clinton and both Bush adminstrations were able to wrok with him to a point, and then control severe problems by using Israel to play him off against. Assad has kept the lid on Kurd-Shia-Sunni troubles in the area that also involve Turkey, Iraq, and Iran. A fundamental Sunni led Islamic regime with ties to Al Quida will not.
Assad does not out and out persecutre his minorities, including Christians. This will not be the case with a fundamental Sunni led Islamic regime with ties to Al Quida.
Assad has been able to maintain peace and the absence of out and out war with Isreal, a fundamental Sunni led Islamic regime with ties to Al Quida will not.
The list goes on. In the current situation there is no (as in zero) vtial US national interest served by attacking or bringing down Assad in the current environment. There are muitiple US national interests that will be severely endangered should Assad fall and Syrai fall into chaos, or have a fundamental Sunni led Islamic regime with ties to Al Quida take over.