Bloodshed and infrastructure destruction tend to help the insurgents. The more the economy is disrupted, the more the organization of government fails, and leaves the field to the insurgents to organize in their place and control the population.
That the government can march into a town and flatten it is less important than whether they can stay in it. If they then need to pull out the fire brigade and go elsewhere to put out the next center of insurgency, it will just result in losing the ground they won.
Resupply of government ammunition is not the critical point. Its whether the government can control the population, and maintain an economy. Iran can’t pay for a war in Syria for very long. I doubt the Syrian state is collecting much in the way of taxes at the moment.
The Saudis and Gulf states however can pay for an insurgency forever.
This thing can go on for years. Or it could collapse tomorrow. Insurgent movements have been defeated in the past. Iran is good at crushing internal opposition. Will they be able to do so inside Syria? Well, they sure raised nine kinds of hell during our involvement in Iraq. 'Stan as well.
I would have thought the Gulf States would have stepped up to the plate by now. More active. But time will tell...