Editing "URL Slug" breaks links

Why when changing a Page's "URL Slug" do any corresponding links not update with the change ?

Would it not be better to have symbolic links, rather than actual hard coded ones ?

Well, they are not also called permalinks for nothing. So  the answer would be no, because they would not be actual URLs which direct to that specific page if the slugs were symbolic. That is one of the reasons that Squarespce includes the ablility to do url mappings, redirect a page's old slug to the new slug. 

This feature looks very complicated, I just expected it to work.

Surely this could be managed by the system ?

No only the internal navigation bars update.