There are a number of environments and languages that have the capability to reload things extensively at runtime. elisp is mostly nice because it is polished, but it isn't necessarily the "best". Erlang has a claim on being better at it, probably. And while most scripting languages are capable of it, that doesn't mean you can just drop one in and go necessarily. It helps to have an environment where that sort of interactivity is first-class and in constant use and testing, which is where the polish comes from.

Erlang the runtime sure. But Emacs is an application with these properties. There is no editor like Emacs for Erlang (some small clones I am sure) and no other user application utilizing this feature.

Smalltalk is a development environment and an applications platform. The development tools are as extensible and user customizable as Emacs. And like Erlang it is also a runtime you can package software for. Any GUI or server software. For Emacs you can probably do the same, but I’m not convince many users would find the GUI familiar.