What if there are no volunteers to sit in the emergency exit row?
The government requires any passengers seated in an exit row to be 15 years of age or older, be willing to assist in an evacuation and have no limitations that would prohibit their assistance.


Source: United

I.e. you won't be given that seat if you can't assist or unwilling.

Personally I always request an aisle emergency [exit] seat near the window. This way the bulky slide is out of my way and I get plenty of legroom. I'm very tall, legroom matters. You'll always find someone willing to switch if they're tall. What I'm saying is, this seat that you hate, others love.

But in your very specific hypothetical scenario of a full flight and wrong seat and unwilling passengers, the government says someone must switch. If the issue causes too much trouble, then someone (who is causing the trouble) will be escorted off the plane more likely, freeing up a space...

... to be immediately filled by someone from the standby list who doesn't mind sitting in the exit row. voretaq7

Have a nice flight.