There are some cases, when we have page on front-end with hardcoded link, even, when link is <inp2:m_Link template="id:12"/> with automatic path building to template. When user deletes this page, then link will be broken until he create new page with same id (which is impossible) or with same name (he never imagines, that he should do it to fix it's problem).
Today is strange approach to create template, that just includes "default_design". This way page becomes system and therefore can't be deleted from structure. I'm against such approach since it's not really understandable, when you look on that template with one include why it was created in first place.
I propose we create new field for categories/sections, that will indicate, that given page can't be deleted without specific advanced permission, which administrators don't have and maybe only super-root will have (or while in debug mode). Also we need to update such column for all system pages and in future use it nor page status (system or not) to determine if it can be deleted.