I board at the last permissible moment (i.e., 10-minutes prior to scheduled departure). I carry very little on-board so I don't care about bin space for one small computer bag.

Someone else is sitting in my assigned seat that I deliberately chose and I am not being offered another similar seat that I am willing to take?

No problem.

"You don't want to move the passenger who poached my seat and want the easy way out by telling me to sit elsewhere? Fine. I will not be taking this flight. I'm sure you'll be able to explain the delay to the company caused by having to find my checked luggage and offload it. Have a nice day."