Most people would be happy to take a camera phone which could go in a purse or small backpack. I pack a tracfone, a heavy desktop replacement laptop, a power brick, and a USB cable to charge the phone from the computer. I'd pack exactly the clothes I need for a few days. When I am in a big city I will browse vintage clothing shops on Hollywood Boulevard or get socks and underwear at Niketown in Manhattan. I throw away as much packaging as possible to reduce weight. I might even buy a bag at my destination and fill it full of plushies from Japan Town in San Francisco or something like that.
Traveling light also makes life easy getting away from the airport, or travelling by bus, train or foot.
If you want to take your skis or are going backpacking in the wilderness you need to take a lot of stuff. In an urban environment, though, there are huge benefits to being light.
It's in everyone's interest for children onboard to have access to entertainment, food, diapers, comfort objects, and clean clothes. That all takes space.
How many nappies do you think the average bundle of joy gets through in an almost 24 hour period? You also need their baby food, a changing matt, baby wipes, nappy rash cream, clothing, books and toys to keep them entertained.
Last time I travelled there was hardly room for a small bag under the seat in front, there's no way 24 hours of supplies for a baby would fit easily. Now what if you have twins?
The twins question is easy. You're only allowed one infant per person. So, if you have twins you and the other adult both have a bag for food, changing mat, wipes, diapers, etc.
If you're already thinking it's too much to spend a few extra bucks to improve the experience for everyone, you probably shouldn't be flying anyway.
I agree on all of your points. However, if there are other factors involed (as I stated, if my child was more difficult), I simply wouldn't travel. Maybe that's not a decision you can make, but I have.
My experience has always been that most kids travel well especially if the parents have prepared.
We sat down and talked about it just a few steps outside. Somebody called the police on us because an officer showed up; the three of us had a good conversation and he left.
If checking a bag gave the same assurance that it would make your destination with you as carrying it on person, fine: make checking a back the only option. But in reality your checked bag is less likely to meet you at your destination than if you'd have just carried it along with you.
How is that different then taking another 20 minutes to wait to get your luggage at the carousel?
Its still 20 minutes of lost productivity for all passengers.