You can't prove anything about a program written in C or FØRTRAN. It's really just Peek and Poke with some syntactic sugar.
Interleaf is very nice. I expect there to be a lot of competition for programs like that.
It is formatted, and I'm tired of using vi. I get really bored.
I got tired of people complaining that it was too hard to use UNIX because the editor was too complicated.
I just don't like to lose what's in the window.
That lack of programmability is probably what ultimately will doom vi. It can't extend its domain.