Well said.

Generally speaking I am a pretty cooperative and non-confrontational person. One exception to this rule is when I feel bullied, which reflexively causes me to stand my ground. OP, I'm sure there was more to the conversation than what you summarized, but the comment about putting your foot on my bag might have gotten my own dander up in that situation. I'm curious, did you try asking first if the passenger would be willing to place their bag in the overhead so you could both enjoy a bit more legroom? I think of this situation more as a negotiation than a matter of entitlement and am wondering whether you may have overplayed your opening bid. I don't think that arriving first and placing your foot in the space automatically gives you priority.