"Provably the most productive" seems to boil down to estimating that Smalltalk is more terse in statements per function point. Incidentally, by that metric Python is only half as productive as Visual Basic, which doesn't seem plausible. No comment about extra test coverage and prod bugs caused by lack of static typing.<p>"Image-based persistence" means your production-ready code and dinking around during prototyping (in a language with frictionless monkey-patching!) are all buried in one big mutable blob. It's often argued the impedance mismatch with version control is what killed Smalltalk.
But where are the libraries? Compare the size of Smalltalk's ecosystem to Java's.<p>Not to mention the various competing flavors of Smalltalk, whose code bases may be incompatible with one another.