Then JavaScript would be used how it is intended to be used and where it is the best tool to do its job. Technical reasons over popularity and management.
Even simpliest webapp functionality like client-side todo list require enormous boilerplate when using only standard library. Standard library just don't have abstractions that are useful in almost every web app.
With vanilla JavaScript you need to take in account the different browser implementations and be unable to target older browsers without newer featurea unless you write a polyfill library, and then it stops being vanilla JavaScript.