But if you think about it, the use case for almost 80% of the people out there that use laptops or desktops is for web browsing and office work (text editing etc) which phones should be able to handle well.
The most successful tablet-laptop attempt thus far has active cooling simply so they don't have to throttle back performance.
It's possible that more energy efficient processors will come about and prove me wrong, but they've been working on the heat dissipation problem for decades now (yes, even in desktops/servers).
This is with a mid-range phone from a couple years ago. I think you're being overly pessimistic.