Or maybe there is wisdom in thinking of applications more along the lines of protocol stacks. A TeX layer implements mechanism, a LaTeX layer does the policy,

Or maybe there is wisdom in thinking of applications more along the lines of protocol stacks. A TeX layer implements mechanism, a LaTeX layer does the policy, and an Overleaf handles the UI chores. (I'm a Spacemacs/org-mode/texlive user, personally.)