Suppose you want to add a ‘plug-in’ such as a new social media share button: often this is going to involve front and back end changes. In my RSS reader this could be a python package with declared entry points so the application could read it, enabling a few more http endpoints and also adding templates to extension points.

In a ‘standard model’ application you need to have an npm package and a Python package and corresponding extension mechanisms on both sides. You can probably make a system that wraps an npm package inside a Python package or maybe the other way around but it will be a struggle.

(The architecture of my current system was inspired by a system I worked in that not only used Scala, Typescript and Python but also Docker such that builds took 20 minutes! They could afford to do that on a venture capitalist’s dime but I can’t.)