Sorry, but I don't agree. Yes, you paid for the footroom, but not necessarily just for your feet. You also paid for 1/150th, or whatever, of the overhead space on the plane - and I suspect your rollaboard takes up a lot more than that. If you were limited to the proportion of the overhead space you paid for, via the same sort of legalistic argument you use for your feet, you'd have to slice your bag in two and gate-check half of it. So, thank your lucky stars that many passengers take less on the plane than you do, put your big bag up there while whispering a silent "thank you" to the folks who donated the space they paid for to you so you can do this, and put the briefcase under the seat in front just like the nice man said.

This has nothing to do with whether or not there was space in the overhead bins when you boarded. Elite status offers many benefits, but the right to be an unreasonable bin hog isn't one of them. "Shared space" is just that. The folks who board after you settle in are not a lower life form.