If I understand correctly, this will FINALLY fix the problem with disappearing peripherals when using multiseat.
For those who do not use multiseat, the problem is that the new InputClass method of adding peripherals is incompatible with multiseat setups. So is hotplugging, which makes ALL X servers listen to ALL peripherals at the same time, breaking multiseat. You have to use InputDevice and explicitly disable hotplugging in order for multiseat to work. If you unplug the mouse or keyboard, your X session becomes deaf, and only a logout will fix it.
So if you accidentally lose the connection to one of your mice or keyboards (this happens quite often when there is some fluctuation on the USB bus, such as when plugging in a USB stick or a camera), you must log out. This happens at least once a day with my cheap mouse, and once a week for one of the three other peripherals.
So this is great news, which should fix MAJOR breakage.
No, you see a working system getting replaced with something that is in development and unnecessary for most use cases, unless you are willing to argue that web servers and render nodes need to be instant on devices, of course.