You appear to have since edited your comment, but the version I replied to referred to being able to choose a seat as the luxury, not flying itself. As I've said elsewhere, flying is either a straight up necessity in some cases and a practical one in others. As I've also said in other places, people without kids can fly without need of choosing their seats.
> But I paid for my seat and if I did pay to sit next to my wife (which isn’t really a big deal for either of us), I would be really pissed if my seat was changed because a parent was too cheap to pay to have an assigned seat.
You can debate on whether or not flying is a necessity, but if we're flying then it's a luxury for you to sit next to your wife but it's a necessity for me to sit next to my infant.