Socialism

It's premature to say that the rapid collapse of Syria actually benefits the west. The status quo was that Russia and Iran had to devote significant resources to prop Syria up. Now they were able to pull out their assets while the west is stuck trying to manage the situation.

Incidentally, there is a RAND paper that warns against this exact scenario. It basically argues that putting Syria under stress is beneficial to the US, but there is a risk of over commitment.

• The United States should oppose any and all policies that seek to partition or divide Syria. A collapsed, divided, or fractured Syrian state would likely contribute to further instability and radicalization in Syria and the region.
• Syria's relatively strong national identity and experience of centralized authority reinforce the prospects for a unified state.
• Lessons from recent conflicts, including U.S. interventions in Iraq and Afghanistan, suggest that postconflict security, governance, and reconstruction in Syria will require viable, centralized state institutions.

Russia and Iran aren’t stupid, and they likely saw that the west was going to flare up the conflict in Syria again. One option was to pour resources into Syria to fight off the jihadists. This would be long and protracted quagmire just as it was last time around. The other options was simply to withdraw and let them take it. These groups all hate each other, and they’re not a cohesive fighting force. They’re already starting to fight each other just days after taking over, and it’s only going to get worse.

The west wants to have a compliant regime in Syria and that requires using coercive methods that will inevitably breed resentment from these groups. This is basically what happened in both Iraq and Afghanistan where the insurgents ultimately turned on their masters.

On top of all that, Israel is now invading Syria in a big way, and they’re unlikely to withdraw. It’s only a matter of time till they start getting attacked, and this will force the west to keep pouring resources to prop them up. In effect, this flips the script on Syria. Instead of Russia and Iran being on the hook propping it up while the west can keep destabilizing it relatively cheaply. It is now the west that’s stuck with a very volatile situation.