To think airborne musical chairs shouldn't be a thing?

[Quimtessence]

Recently, traveling solo, and as I settled into my window seat, got comfy and the airplane doors finally closed, I realised that on this 90% full flight the two seats next to me remained unoccupied. Yippee! This is a rare win and as it was a 6 hour overnight flight where sleep was expected, very welcome that I’d be able to stretch out. Also no one to wake and clamber over if I needed the bathroom. That is until nearly an hour into the flight a bloke from one of the middle seats got up and without a by your leave, plonked his arse into one of the seats next to me and remained there for the rest of the journey, watching a film!
Exact same thing happened on the flight home, except this time the flight was only 60% full and during daytime, so not quite so galling. This time, a little old lady, without any acknowledgement or word to me sat herself down in one of the empty seats next to me and stayed there for the duration.
Who is being unreasonable here? Are unoccupied seats fair game for anyone wanting to upgrade from their assigned one or am I the one with a sense of entitlement thinking I have some sort of ownership of empty seats next to me and should be left to enjoy the rare luck I thought I had been handed?

The last long haul flight I took was half empty, and the pilot announced in between take off and landing anyone was free to move if they required.

Yep, not uncommon.

It's often a stated/published in the manuals policy at some airlines that crew allow seat moves (within the same cabin 🙄) after take-off, though sometimes with the caveat that passengers should return to their original seats for approach and landing (due aircraft trim).

YABU. I'd move from a middle seat between two people to an unoccupied seat at an aisle end in a heartbeat and I'd expect others to do the same. I've purchased one seat, not three. I fly a lot and have had this happen multiple times and never thought anything of it.

YABU and using the phrase “little old lady” doesn’t endear me to your argument either.

You wanted the unassigned seats for yourself but someone else got them first? But you thought that you somehow had a claim on them?

without a by your leave, plonked his arse into one of the seats next to me and remained there for the rest of the journey, watching a film

You sound quite entitled. You wanted two free seats to yourself and begrudged this man taking one of them? And felt as though he should have asked your permission?