I justified that with the word complex. In my mind, a complex app is one that is built and maintained by a team over a long period of time. I don't know what you're doing to maintain complex apps between a dozen (or couple dozen) developers on with <script> includes, ES5, and pure CSS & JQuery...but I'd really like to hear it in a blog post of some kind. I've worked on large code bases with those tools and it was a nightmarish experience just to upgrade to the latest version of JQuery. If you're fine churning your wheels doing all that stuff manually rather than depending on NPM...sure I suppose you can get by and waste a lot of precious time in the process. If you care about user experience and developer experience, however, build tools are absolutely essential.