If no one has paid the extra fee to sit there, the gate agents and/or cabin crew will look at the manifest and "upgrade" frequent fliers, tall people, whatever, to make sure these seats are filled.
For light relief....
As I understand it if one feels "unable or unwilling" to operate emergency exit then they "must inform cabin staff".
So if the entire cabin wanted to screw the crew of a particular flight (lets say they have pushed back and are sitting on the ground for 2 hours in high temperatures) they could organise themselves (!) and refuse to sit in an exit row.
Aircraft would be forced to return to gate with zero consequences for passengers?
Mickjoebill