Why would structural editing be a dead end? It has nothing to do with storage format. At least the meaning of the term I am familiar with, is about how you navigate and manipulate semantic units of code, instead of manipulating characters of the code, for example pressing some shortcut keys to invert nesting of AST nodes, or wrap an expression inside another, or change the order of expressions, all at the pressing of a button or key combo. I think you might be referring to something else or a different definition of the term.