ES modules mean you don't need to bundle your code; you just include your index.js in HTML, and all 30,000 JS files of your project come to the user's browser without trouble or delay (let's wish them luck, lol). Since you're bundling, it doesn't matter which module type you use; CJS has worked with code splitting perfectly for over 10 years. However, it's a pain every time you try to import a CJS library in your ESM or vice versa. The truth is, you can't just drop all legacy CJS packages in most real-world projects.