Let's compare it to some other "unnecessary" thing, actual containers:
You can put stuff on ships without them, but turns out that once you start using them, just the fact that everything is standardized gives you insane benefits.
Of course you could reimplement each part of Docker differently. Of course it's not magic. Nothing is magic about a metal box, and yet that metal box completely revolutionized shipping.
What is certain is that systems like Debian and Fedora are not going away. Your Docker images couldn't be built without them, after all. And the other tools mentioned in the article, too, are not going anywhere. So why don't you just standardize on the real underlying platform?
Is docker the silver bullet for this? No, there's tons of other options. But "everyone should use Debian and Fedora" isn't a realistic standardisation.
But k8s still uses the same concepts used with docker-style 'containers', with registries, layered, isolated filesystem, separate networking, ... all in one box.
Docker was just a big nudge in the direction of democratising scalable cloud infrastructure/application stacks. Yes it can be used for other stuff, but that's what it's extremely good at.