Whilst i agree with you about previous western interventions this one i feel is a lot different and one where there should have been intervention, Assad was on the verge of being toppled just a bit more support was required, then of course the Iranians and Russians ramped up their support which leaves us where we are now. Even if Assad wins the next few battles he will never win in the long run and will never rule in peace again, he's killed far too many for that to happen now, another rebellion is just around the corner, so i do not see the end game or benefit in propping him up.
RE foreign fighters, there are an estimated 27k foreign fighters in Syria and Iraq, the majority of this number are ISIS fighters, who don't actually fight the regime, this number alone is not enough to fight the Regime, Iran and Russia, there is, contrary to the Regime propaganda genuine resistance from Syrian people who do not have appalling ideologies so they should get our help.