So my Annoying Brother-in-Law and Sister-in-Law recently went on a 7 hour plane trip with their 19 month old girl. To save money, they did not get the toddler her own seat, figuring they would take turns holding her in their laps for 7 hours.
On the flight over, the person next to them decided to upgrade to an empty seat in first class (probably took one look at my very active niece squirming in her parents arms and thought "Oh hell, no!") so they were left with an empty seat next to them, that the toddler used.
On the way back, though, the plane was full, so they had to hold the toddler the whole time. And apparently they had an aisle and a middle seat, but my ASiL wanted the window seat and got extremely annoyed that the person sitting there wouldn't trade with them so the toddler could look out the window (or so she says. I suspect she really wanted the window seat so that my ASiL could sleep against the wall of the plane).
So she wanted the person who actually paid for the window seat to give it up, to make it more convenient for the part of 3 who only bought 2 seats? And if the person had given up their seat and taken the aisle seat, then they would have been stuck having to get out of the way every 10 minutes when the toddler decided she wanted to get up and wander the plane.
You're already inconveniencing your seat mate by bringing your noisy, active 19 month old on the plane. Where do you get off expecting them to bend over backwards to accommodate you because you chose to bring a small child on a holiday to Scandinavia that the kid is never going to remember anyways?
These people, I can't even...