I agree the difference is emacs is done such that it is all exposed to all developers. There are no special parts, as it were. Such that a hello world looks like (defun my-ext () (message "hello, world")). If binding this to a key, you simply need to add (interactive) after the argument list.
Obviously, things ramp up quickly, but the point is it isn't some special framework to make extensions. It is just a function.
As you say, Emacs has no special framework for extensions - it's all just functions and variables, plus a bunch of higher-level concepts (e.g. minor and major modes, customize, autoloads) to pick and choose from.
Tradeoffs all the way down :) If we didn't have them we wouldn't be engineers...
This is an odd shift of the word "stable" in software. For a long time, folks took stable to mean simply "doesn't crash." But, it also needs to mean "remains unchanged" if you want to use it as a foundation of other work.
Of course, I say that, and have to ack that the web itself has proven a major exception to this supposed rule. Nothing has been stable in that entire landscape, but it has continued to grow at an astonishing pace.
The extension model is the same in VS Code, all the author needs to do is write basic JS (or TS). VS Code core does the heavy lifting of creating the extension host process to run that code and exposing the `vscode` proxy object to the extension's code, which enables communication between the extension and the renderer in a manner that appears identical to as if the `vscode` were a simple object. See the minimal hello world sample [0] for the basic case.
End of the day it's a tradeoff between latency and throughput, emacs chooses latency, vs code chooses throughput. Both have their ups and downs, largely dependent on the size and frequency of the task at hand.
[0] https://github.com/microsoft/vscode-extension-samples/blob/m...