You forgot COS.

Anyway, I don't know if I'd use the words "MAKE YOU" do it, but yes, I think they are within their purview to ensure that te big bags go up top and the small ones down below for maximum storage and efficiency.

I get your point about foot space you say you paid for. However, on the other side of the coin, you and every other passenger ALSO paid for the right to carry on two items, and those items have to go somewhere. This MIGHT sound like I am coming down on you, but that is totally NOT my intent, but here goes: Your "right" to the footspace you paid for doesn't trump someone else's "right" to have a carry-on inside the aircraft. Since the flight was said to be full, I think the FA was going with the contingency plan to make sure all passengers get a similar minimum combination of comfort AND carry-on without one person getting it all and the other person getting none.

If you had been a late boarder and had to gate check an item that should not be out of your possession because someone else didn't feel they ought to give up their foot space, I'd bet you'd be pretty angry. I know I would.