Everything is "just programming" but deep semantic heuristics from an IDE can't be done reasonably in Emacs. Most of the big features require keeping a full AST model of the project in RAM. That would make emacs/vi really slow.
IDEs are 100% extensible via plugins. Wrote a few of those myself.
> Most of the big features require keeping a full AST model of the project in RAM. That would make emacs/vi really slow.
That's what LSP does, and emacs supports LSP. Heck, it even has two packages implementing support for LSP.
For that matter, I don't see any reason why Emacs would be any worse at keeping a full AST in RAM that is an IDE. Emacs Lisp is not the most efficient runtime out there, but it's acceptable.
> IDEs are 100% extensible via plugins.
Plugins are not 100% extensibility: they only enable one to extend the IDE in the ways the plugin system was designed to allow. Emacs, OTOH, enables one to extend or replace all editor functionality implemented in Lisp. I am not aware of any other IDE which offers, for example, all of the following: multiple email clients, a net news client, an IRC client, a Slack client, multiple web browsers, a Git UI, a spreadsheet, a personal organiser, multiple calendaring systems, a computer algebra system, multiple file managers, multiple shells and terminal emulators, an RSS client, a music player and Tetris.
I haven't written LSP so I'll reserve the opinion at that. What I understood about emacs is that it doesn't have a concept of a project so I don't see how it can form dependencies that are more sophisticated. But maybe I don't understand something about that.
I can totally write these sort of plugins for any IDE I worked with. Notice that all of these capabilities are symbols of a bygone time where you stuck everything into an editor. There's no motivation for a plugin developer to do these sort of things. Instead we have plugins like Lightrun, TabNine/Copilot, etc. Stuff that's actually useful for development.
JetBrains (and some other IDEs) have a builtin file manager, terminal, etc. These make some sense for development.
Extensibility via plugins is quite a bit different from extending the editor as you use it. Emacs admits the latter, and useful functionality can be added in a page or less of Emacs Lisp.
I tried writing a plugin for Visual Studio Code. It made me want to throw furniture.
That is true. IDE plugins are a nightmare to write and VSCode is by far the worst to write. Still, JetBrains is remarkably extensible and mature, I was able to integrate things very deeply in the editor. The downside is writing in Swing which is pretty long in the tooth by now.
Easy extensibility. Read email. Run a calculator. Run Lisp …
Meanwhile, there is literally no feature any IDE has which Emacs cannot do. It's just a simple matter of programming.