They’re different tools. If I were building a JS server for a backend, I’d use Express. Next gives you things like server side rendering and static site generation out of the box, and abstracts/blurs the line between server and client code through its paradigms. For better or for worse.

The deploy infrastructure is quite nice. Nextjs is surprisingly low config, even if you forego the Vercel deployment route it’s not difficult to generate a static site or docker container