One of the mantras of functional programming is get your data structure right, and everything else falls into place. As an experienced functional programmer, I see this in practice every time I program--even a slightly "wrong" type pollutes the code dramatically.
The idea then with structure/tree/semantic editors is two-fold.
First, text is obviously the wrong the wrong data structure for programs, so even if people manage to provide fairly good support with the "modern IDE", the effort it takes is staggering. And still details like files, character encodings, god damn tabs vs. spaces, and terminals leak through. One can't help but wonder how much farther they would get were their efforts directed more efficiently. Look at this https://github.com/yinwang0/ydiff for example, blows text diff out of the water.
Second the idea is maybe text is the "wrong data structure" for programmers too, not just tool-smiths. I admit there are bunch more psychological/neurological aspects that make the answer less obvious. But I see no reason why not to keep an open mind until a polished tree editor exists for empirical testing. Even if text is in someways better, the improved functionality a tree editor offers could offset that dramatically.