Take a step back to a larger scale ... Light destructively interferes in some areas but constructively interferes in others. Net energy transfer is the same.
Intensity measures energy arriving per unit area per unit time. An interference pattern consists of bright fringes and dark fringes. Whilst intensity is reduced at dark fringes it is increased at bright fringes. But consider the whole pattern and net intensity will be the same as that for the light energy arriving when there is no interference pattern (i.e. same light source but no double slit).