while i agree entirely that there's nobody involved who i'd like to see win, there are probably risks and downsides to doing nothing as well - certainly the obvious ones are the eternal continuation of a war that carries great risks of spilling over Syria's borders, an eternal refugee problem on Syria's borders, and the ever hardening radicalisation of the Syrian population who see themselves as being left to the tender mercies of both the Assad regime and the AQ inspired fundies and developing something of a grudge towards the west because of it.
added to which of course is that for every month this war continues, about 6,000 more people die.
in order to defeat the Assad regime the opposition needs lots and lots of Anti-Tank Guided Weapons - like Javelin - lots of MANPADS like Starstreak, and lots of Artillery. giving them MANPADS and ATGW runs a very real risk of seeing them turn up at Heathrow in a years time with 500 dead on a burning 747.
if we wish - for purely selfish or purely humanitarian reasons - to see the war ended as quickly as possible, but without the risks of 'blowback', then we have to keep the weapons under our control - which means us using them. this is not an attractive option.
air strikes on Syrian government targets carries the 'upsides' of keeping the targetting and weapons under our control, of reducing - for example - the ability of the Syrian Air Force to attck rebel forces, and it would mean the Syrian government would have to divert resources its currently using to fight the rebels to Air Defence tasks; some of the reports that i've read suggest that the Syrian AD infrastructure has been hollowed out to provide more ground forces.
all the options available - including doing nothing - are shit, they all carry big downsides for the Syrians and for us, and none of them provide attractive outcomes.