I acknowledge that having the point always on-screen has benefits too
(though e.g. a fringe indicator could "point towards the offscreen
point" to address some concerns there...), but what if I'm just
scrolling up to look at something? Martin's hack ensured that I can
scroll away to have a look, and the point will reliably be where I left
it when I scrolled back.
The functions you're talking about are fulfilled in Emacs by the mark,
not the point. I frequently scoll a bit to look at things. Then I
scroll back to get where I was, unless I want to stay where I've got to.
The point is, by your mechanism, having scrolled away from point (leaving
point off the screen), how do you then indicate where on the screen you
want point to be set to, and how do you set point there? The general
Emacs answer is that you indicate a position by setting point. Hmm.
We're going round in circles. (You're presumably going to say "you click
the mouse to set point." This violates the Emacs principle that
everything must be doable without a mouse.)