> The only reason this doesn't exist is simply that nobody has decided to build it yet.

It also takes a lot of time and effort, and so needs to be funded along with minimal interference from the bureaucracy of a business. Smalltalk and Hypercard came out of precisely those kind of environments. Today the goals of businesses are much different -- much more short term and quarterly focused. I don't have much faith that FOSS can come up with it either, since so much depends on part time work of loosely connected people.

I hope I'm wrong though!