One of the aims of a diplomatic solution would be to 'drain the swamp', that is, peel away those who have been supporting IS, either covertly or openly. If this could be done, they would shrink a lot. This includes those countries who send finance and arms, or buy oil, and those Sunni leaders and tribes who support IS.,
It's clear that this can be done, as it was done in Iraq with Al Qaeda, who were initially supported by various Sunni leaders. But eventually, they fought against AQ. Of course, this all went pear-shaped, and IS emerged.
I don't see why Corbyn's ideas are partisan either. They sound sane to me.