The title more or less says it all, but it bears repeating as many times as you can stand: a software project should only use the tools it truly needs, rather than bringing in the latest and greatest cool new framework when all that was really _needed_ was a basic static website. Although the overuse of React is not the only case where this occurs, it makes for a good example.
<script> tag or a single
scripts.js file. Loading ReactJS every time will make the site significantly slower and consume resources that shouldn't be required, and it risks greatly reducing the number of supported browsers.
This is probably the most boring, generic blog post that I'm going to write here, but that's because it's a message that bears repeating over and over and over again: don't use a tool just because it's the cool new thing to use! There may be something better for the job, which will consume fewer resources and allow you to build your project faster, while being easier to use for a large subset of your users. Don't overcomplicate things!