I'd argue the whole point of React is to enable the modularization of components written in Javascript.

I realized as soon as 1997 that if I wrote some component in Javascript I wanted to stick into a web page I might want to stick two or three of those into a web page and to do that I have to keep the state for those separate, be able to reference the form elements (didn't have access to DOM elements!) with a unique name, etc.

React goes past that and lets you nest components into other components. In some point that's "managing complexity" and not "introducing complexity", except for the fact that when you can easily incorporate 35 third party components into your application that is 35 components each of which introduces its own complexity of one kind or another -- some of the complexity of "modern Javascript" is the complexity of the build system and the framework, but the build system and framework system let you build bigger systems that have bigger problems.

Software is a Jeavons paradox. When you use a library (time efficiency) you are compelled to add more features (complexity)