Honestly, I will never wrap my head around how people can happily bring infants on any flight where you can expect people to try and sleep, it's incredibly lucky if they don't spend some of it screaming their heads off—I would be mortified if my choices were preventing hundreds of people from sleeping. But I'm not going to rant too hard about that.

Why on earth hasn't any airline started marketing adult-only flights?

It seems like a complete no brainer to me, I would choose it every time and pay extra for it.

Disclaimer: I may or may not be on a 36h day with only an hour of sleep right now