Mutable Software Is Creative
The easier software is to modify, the more interesting its ecosystem becomes.
Good examples of this include Emacs, Atom and Pharo.
why hasn't this won?
Mutable software doesn't scale.
If you can modify it, you can debug everything, fine-tune everything, and build something that meets your needs.
It's really hard to succeed when you have multiple maintainers, projects of varying quality, and shared APIs or resources.
JS in the browser or IDEs like VS Code succeed because they take a more hostile view of extension. This limits their applicability, but makes the platform more robust.