Be very careful if you're going to hold the youngest in a sling, especially if it's before they can hold their head up/roll. I know a woman who accidentally suffocated her 6 week old whilst chasing her toddler, it was a horrific tragedy. Slings are only good for short periods when babies are tiny - a flat pram is much better.
That's a very sad story, but if you follow safe slinging guidelines, and use basic awareness of the child you are carrying, its fine.
Your are also hugely contradictory saying that you should be wary using them when they are tiny and can't support their heads, but also that they are also only useful for tiny babies and short periods of time.
Many of us (inc me) use them for several years - some are literally designed for pre school plus age children.
Obviously, parents need to be aware that their breathing isn't obstructed by their position, but that goes for a buggy or carseat -and with a sling on your front (which is where using babies go) your are legally choose enough to kiss them and feel their breath. It's very easy to keep an eye on them.
If the OP doesn't want to use a sling, fine, but your post is full of scaremongering.