It doesn't take that much space. Certainly not more space than that you can fit under the seat in front of you.
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.