On the desktop you can press the “End” key to jump directly to the last message.

Navigating with PageUp and PageDown in a Discourse thread feels smoother than in a phpBB where you eventually have to hunt for the “next page” link.

You can jump to any point in the thread with the Discourse thread scroll bar too.

which goes back to the confusing UI. why should there be a different scroll bar to use than the standard one?

... because it's not possible to load the whole thread at once, so the normal scroll bar will be jumping up and down as you scroll to load content on demand. And really, if you want to jump to a particular post in a long thread, the special scroll bar is easier than pages anyways because it's continuous.

Really, the scroll bar IS a fresh take on the page list, except it's actually useful for navigating the thread and doesn't force readers to deal with the useless concept of numbered pages when such arbitrary divisions are no longer the primary way of navigating the thread. Once you choose to not implement explicit pages (and accept that it's not possible to load the whole thread in one request), the thread scroller is a natural consequence.

There is no usable standard scroll bar on mobile.

Not sure if I am reading you correctly, but you have to tap on the x/y label to see the thread scroll bar on mobile.

I meant the standard UI scroll bar. On iOS and Android you have no usable scroll bar.

It is only a location indicator (which is of course completely useless on infinite scroll pages).