Pretty cool that Linus Torvalds invented a completely distributed version control system and 20 years later we all use it to store our code in a single place.
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Maybe if Git had native support for PRs and issues this wouldn't have happened. (And yes I'm aware of git send-email etc.)
And those updates are properly tracked by your version control, not done jankily by editing a commit and rebasing and force pushing.
Though your 'stack of commits' is more like a 'stack of diffs' (at least in the Git sense of commit-is-a-snapshot-of-the-whole-repo.)
And actually, for better or worse, a PR branch is more expressive than a 'stack of commits', exactly for the reason you suggest: a PR branch can also contain merge commits, it doesn't have to be a linear chain.
What you'd need to do to support 'stacked diffs' or 'stacks of commits' is to keep a history of what you mutable pointers were doing. It's a 'meta-git'.
Once you have that, you could build all the convenient operations on top of that abstraction.
Btw, if memory serves right, git has some plugins (or whatever these things are called, plugins or so?) for implementing the stack of commits concept.
I don't know whether they go the 'meta-git' route that I sketched, or whether they do something less abstract.