Iraq, Syria and all the rest of the countries in the area are an artificial construction by the French and British at the end of WWI. The only thing holding those countries together was brutal oppression. As that fades, there's going to be chaos as the borders are remade into a more natural state.
Complicating that even further is Israel (for obvious reasons) and Turkey. Turkey's a US ally and NATO member, but a large chunk of "Kurdistan" should be carved out of their territory. And Turkey is not interested in doing that.
So the goal of the bombing and training isn't to utterly annihilate ISIS. Despite what the "I continuously crap my pants" caucus claims, or the media massively oversimplifies.
The goal is to weaken them so that they stop conquering, while trying to strengthen some less brutal group who will be able to represent the Sunnis. That way when the borders get redrawn, they might be somewhat stable.
As a result, ISIS not being destroyed in 18 months really doesn't matter. The problem will be if the Sunnis will not get behind a less brutal group. If the chaos in the not-as-violent Sunni groups continues, there really isn't anything more that can be done. The locals will wash the sand with blood while they fight the war that has been brewing for the last 100 years. US bombs won't particularly matter in that situation.
What will matter is the Turkey problem. Imagine an ISIS that moves into Turkey. Turkey's NATO status obligates us to put boots on the ground to fight off an ISIS invasion. That is the scenario that the current plan is trying to prevent. Since the bombings seem to have already stalled ISIS's advance, it seems pretty unlikely that they will invade Turkey any time soon.
Couple that with hardening resistance from the Kurds and the Iraqis (their government and army actually give a fuck about the Shi'ite areas of Iraq), and it looks like the worst-case scenario is ISIS forms a new country from parts of Iraq and Syria. Not exactly positive from our perspective, but livable in the short-run. And 18 months is probably too short of a timeline for that to occur.
Which is a very long winded way to say I don't support "boots on the ground" despite supporting the bombing/training plan.
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