systemd is not an init system. That it has one, it out of necessity. To manage a system you have to manage what runs on it, to manage that you have to manage what gets started and stopped, and so you have to manage boot and shutdown. And now, here's your nametag, there's the counter, don't forget to smile, you're the new init system!

udev doesn't have to be there, but when a handful of people maintain critical parts of the Linux ecosystem and they saw ways to make their maintenance work more effective, they went ahead and made their work more efficient.

I mean, criticizing systemd/udev/Debian/Ubuntu/Arch on how they did stuff, and the technical merits of how things are is completely fair, but criticizing "why" is strange. They did because they saw that it will help them, their projects, their users. (And they probably understood very well that it won't please all of their users.)