History is full of revolutionary groups who promised moderation but delivered extremism, either due to circumstances or their own inclinations. As a result, it was appropriate for the Israelis to destroy the Syrian military assets (they probably regret not taking advantage of the opportunity during the civil war) and for the Americans to bomb IS.
But it would be a mistake not to explore the opportunity to work with the new government, whatever it may be. If the absolute worst case occurs–an IS takeover, which is highly unlikely–the regime will be surrounded by enemies that are far more powerful than it is, so containing it will not be overly difficult. On the other hand, it is certain that the government’s principal enemies for the foreseeable future will be the remnants of the Assad regime and its allies–Hezbollah, Iran, and Russia. Why not work together against the common adversaries?