The most important accomplishment of Da’esh, in my opinion, and the thing which argues the most convincingly that they are a genuine organization and not the creation of Western Intelligence, is their highly successful system of economic self-reliance.
They have created a model for financial independence, self-funding, and have taken their territory almost entirely off the grid of multinational corporate control. Whether it is sustainable or not remains to be seen, but it is a remarkable achievement.
I suspect that their project will not ultimately achieve its goals, but what they have pioneered in terms of economic sovereignty will inspire future strategists to develop more workable models that more fully navigate the realities of global power dynamics.
The biggest threat Da’esh poses to the West is not terrorism; that threat is so miniscule it doesn’t even merit mention. The danger of Da’esh is their economic model, and what it would mean if Muslim countries adapt it effectively.