As the parent of a 9 year old who is well traveled, I still wouldn’t sit so far away that I couldn’t see him and spring into action if needed. I’ve never sat in a different cabin. And while J is, I agree, basically wasted on him, at least he is near me.
My kid is well behaved, courteous, and fairly independent. But sometimes needs help with a difficult seat belt or a tray table that’s stuck or reaching the light button. That’s my responsibility as a parent. And frankly if I had to sit next to someone else’s child and converse or assist regularly, I’d be very annoyed. Parenting can be exhausting, especially during travel, and I do think it’s inappropriate to put that on a stranger.
I’d also say that a child is different than an annoying seat mate. Kids have different needs and certainly don’t have the life experiences of an adult.