I have experienced working on a messy application codebase (jQuery + Knockout.js) which was 7-8 years old and seemingly was hastily made by on contract of another company. Had to refactor it to ES6 (dynamic imports) -needless to say it was a big pain with unpredictable barely any performance improvement as compared to refactoring on another codebase.

jQuery is a good tool for simple one page applications but not designed for complex web apps. It is harder to code cleanly in it as compared to React.