By getting rid of all these different ports, they are making their product simpler, while still capable of interacting with devices that use them (via dongles). This means they have to build less into them making them also lighter, more portable and requiring a standard way to connect other devices.
Dongles are a pain. Yes they are. But also one can argue that in a lot of situations, maybe the majority, if you are in need to use external devices, you're probably in a situation where your laptop is on the top of a desk, in which case maybe even using some kind of a HUB makes more sense (this applies both for home, office and meeting rooms).
So why aren’t they doing this in 5-10 years time? Well, because by doing this (Apple and others), they’ll speed things up. They’ll create more need for devices compatible with USB-C, which means companies will start making peripherals for that market.
However, think by now we’ve all understood that Apple is aiming for a wireless future. The last compact camera I bought mid-2015 already had Wi-Fi. Headphones are moving wireless (hopefully in a safe way, since there is a reasonable concern about the health effects). Wireless peripherals already exist. The reason why they still have USB-C is probably mostly because of charging, with the nice side effect of keeping other devices able to connect via a simple dongle (rather than a wireless HUB).
I’m also not a fan of wireless keyboards and mice. Mostly because of the hassle of managing batteries/running out of juice and not finding other batteries. A good move from Apple would be to solve that problem: wireless charging becoming the norm instead of an expensive add-on, with longer lasting batteries, with the dream being on-the-fly wireless power feeding.
tl;dr - Apple wants to make things simpler, lighter, more portable and they are not afraid of putting the weight of their whole range of products behind it and taking the plunge. There will be some pain. We’ll hopefully end up with a better experience after the transition period.