At what point do we add active cooling to phones?

I don't mean that as a joke, there are some really interesting systems being developed (which are functional) that are extremely tiny that can move air with no moving parts, they're being developed for exactly this sort of use in electronics.

So, no need for a fan that wears down, creates noise, etc.

The biggest issue would be keeping the air channel free of dust and debris, if they can figure that out, then adding a few millimetres of thickness and a small weight increase in order to accommodate future higher capability processors seems entirely acceptable.

Come to think of it, who's to say the air needs to come from outside the phone? You could have a closed system, with air channels moving throughout the phone's metal chassis, acting a little like a heat pump, to better distribute the heat generated in one spot across the entire phone surface area for dissipation.

Phones already use their chassis for cooling, this would just significantly increase the efficiency with no exterior changes. Hmm....