It's less that Russia supports Assad, it's more of not wanting another West-supported intervention and regime change.
Agreed. But this isn't worth starting World War III over. Russia might pitch a fit, but they won't be sending any help to Assad.
I'm slightly more optimistic about the next regime. For a start, the Syrian Free Army is powerful enough that unless Turkey wants to fight them, the new regime will be led by someone from the SFA. They'll likely resent America for not helping them against Assad, but they'll hate Iran and Hezbollah a lot more.
If Turkey does go in, I think the US should pitch in as well. Given that Syria's probably out of range for the French or British air forces (barring the use of Turkish or Israeli airbases), I think the US should take the lead more this time. Park a carrier off the coast, and make the Syrian air force go away.
I've also wondered if Israel should consider getting involved. On the one hand, it would hand Assad a major propaganda weapon. On the other hand, it might be too late for that. And if you want an idea of what Israel can do to Syria, look up the Six Day War, then consider that Israel would not be worrying about Jordan or Egypt. Most importantly, it might be the key to finally bringing about peace between Israel and Syria (they're still technically at war). If Israel helped out in a big way, the new regime would probably be quite willing to come to the table over the Golan Heights.