I think this was a plausible position two years ago; now, I think that Syria is doomed to a long and bloody civil war and propping up Assad is neither a credible nor desirable outcome. Unfortunately there are no reasonable alternatives, either. Which is why I think that we should simply stay out of Syria altogether until we have a plan that makes sense.
I have heard that Assad offered the US a deal, via the Russians, that would have involved him standing down and going into exile and the regime continuing with some accommodations and assurances three years ago. The US apparently turned this deal down because, at the time, they believed they could easily push Assad out (and had the UK joined a US-led force at that time this would probably have happened for better or worse). This was, I think, the key mistake in foreign involvement. A negotiated stand down of this type would have been the best possible outcome for the civil war, but the window of opportunity is long since passed.
I thought Hillary Benn's speech in the second commons vote on military action in Syria (this time backing the opposite side to the first vote) was the clearest example of the dire thinking over Syria. They're mean! We must do something! In contrast to the battle against ISIS in Iraq where we have clear allies (the Iraqi government), clear targets (ISIS), and a clear end aim (the restoration of Iraqi territorial sovereignty); any involvement in Syria is directionless rubbish since we have none of these things. We're effectively backing Assad by proxy whilst condemning Russia for backing him in fact, and deriding him and his regime in public.
