The size of the space under the seat is actually a few inches larger than the maximum size for a personal item on many planes. I generally travel with a M/L backpack (depending on how much is packed in it) and a small purse. My backpack is larger than the dimensions of the a personal item, and besides, my purse is my personal item. I have no qualms about putting my backpack in the overhead, despite the fact that I could probably stuff it and my purse under the seat. Why should I be punished for packing light? Should I go out and buy a larger bag, which would take up more space in the overhead just so people will think I 'deserve' to use that space?
This is all kind of off-topic though, I see nothing wrong in OP's behavior of asking about moving luggage. Having multiple people place individual items in a space is not an efficient packing algorithm, and most the the FAs I've seen are very good about tetrising the bags up there into a better arrangement. I'm kind of amazed this FA was so unhelpful, and then to escalate all the way to the captain! Agreed that goes beyond a bad day, that is not a person who is suited to their career choice.