@ossobuco,
I don't agree that adults chomping on potato chips is just as irritating as a fussy baby and I don't think they belong at the back of the plane. I've travelled on airplanes quite a bit and there's usually at least one, if not several, crying child. And they're not just infants - toddlers have loud cries and wails, too. Of course it's irritating, but there's nothing you can do. Their parents cannot always shush a child.
Practically speaking, you can be 20 seats away from a howling kid and still hear him loud and clear so what good would it do to put them a few rows behind? That doesn't even make sense.
I've often found loud adults or teens talking more irritating than babies. And then there's incessantly talking older children.
It's one of those things you have to suck up with air (or bus) travel. I just plug in my ear phones and focus on the music or tv or my book and within minutes I'm able to tune them out.