The promise of extensibility is that we can build levels upon levels of code starting with existing languages until we reach true artificial intelligence and then sit back and drink wine coolers while the AIs rule over us;)
Object oriented languages (and other kinds of extensibility) were supposed to make it so that we could build levels upon levels of objects. These levels would be available to everyone so that people could keep contributing levels.
I don't see this happening. Yes we have libraries, but we have no clearly defined levels to build on. I go to work every day and work with my object oriented language. Most of my time is spent writing loops and conditions and filling variables and so on. I am in the education industry and I should have an 'education level' to code from, not just language constructs. I have heard that some people have managed to get perhaps a level up, so that they work with things that were written in these languages. Still, nothing has stood out to the extent that people use such and such 'level'.
Outside of user interfaces, there is no community devoted to even one built-up level. And let's face it, as important as UI is, it isn't what the program really does. It displays things and accepts interaction, and yet it is not the core of the program. I see attempts in other areas, but little success.
On the whole, our programs still mainly just push data around, perhaps analyze it, store it, allow access to it, but not much else.
Why is this?
Why haven't we built a major well-known level, then built another major well-known level on top of that, then built another on top of that?
Where is the advancement in software development?
Why hasn't this happened?