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I wouldn't recommend not intervening, and I can't say I've heard any serious commentators suggest not intervening. The aim of those opposed to nonsensical air campaigns is simply to have a proper UN mandate for intervention, to restore some semblance of international law in these cases.
The idea 'we bomb and a ground-based guerrilla outfit whom we support will sort out the aftermath' simply wouldn't work. That would only mean that civil war would continue to simmer under the surface even if it could ever end (there is no guerrilla campaign in the world which isn't bitterly opposed by some).
A negotiated peace settlement which brings lasting peace and stability and would be accepted by all could only be brokered by the UN. Needless to say, certain Western powers have no interest in bringing stability to the region, as this would mean an increased oil price.
However that may be (and I'm obviously not naïve about the UN's capabilities), it is very important to move away from the Bush Jr. way of starting illegal wars just to remove a formerly compliant American puppet governor who knew too much about the Bushes and their cronies--oligarchy at its worst.
Let's try the other side of the coin.
Let's say the great powers, including Russia, all just backed off and left Daesh alone. What would you expect to happen?
Let's also say the west backed off and Russia didn't. What would you expect to happen then?
Edit to add - I'm not expressing an opinion for or against, I just want to know what people think would be the outcome of not intervening.