And the quill doesn't perform the same function?

What you're really talking about is that if you buy a bike that doesn't fit you and has a threadless fork that is cut too short, a high angle stem will accomplish what a quill stem used to.

Which is true, but it all starts with a bike that doesn't quite fit.

Thank you for telling me what I’m “really talking about.” But where did I say “a quill doesn’t perform the same function” as a threadless stem? We’re talking about two different systems for bikes specifically designed for two different systems. What you really seem to be doing is creating an argument when none existed in the first place.

And ........ I told you so. No matter how low your expectations are, there's always someone who can meet them.

My current state:

For people who like a very upright riding position and want a lot of rise without a lot of reach, I wonder if there's some potential weight savings to threaded. Just the quill stem has to extend very long above the headset in that case. Basically a single pipe.

With threadless, both the steerer (or worse, the steerer plus steerer extension) have to extend very long above the headset, but then also the spacers surrounding those have to proceed all the way up from the headset to where the bottom of the stem is clamped. So a double pipe.