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.
No, I know that. But we do have an example of something that does: the web browser.