Come the BABLR side. We have cookies!

In all seriousness this is being done. By me.

I would say structural editing is not a dead end, because as you mention projects like Unison and Smalltalk show us that storing structures is compatible with having syntax.

The real problem is that we need a common way of storing parse tree structures so that we can build a semantic editor that works on the syntax of many programming languages

I think neither Unison nor Smalltalk use structural editing, though.

[edit] on the level of a code in a function at least.