The Gate Agent Upgrade Dilemma

I was on a flight from ORD to SFO yesterday. I had chosen my seat It was an exit row aisle seat.

At the gate the agent called my name. Offered me a free upgrade to first class. This sounded like a thing but the seat they were offering me was a middle seat. It was also in the row of first class.

I said thank you to I did not want the seat. The man who was behind me in line took the seat away. He looked at me like I had made a decision. My reason for saying no was simple. I would rather sit in an aisle seat for than four hours than be stuck in a middle seat. This is true even if the food is better in class. I like being able to get up and move around when I want to.