Next.js is a a fully functional backend. Its not Next's fault - I dislike it for other reasons, but this was not a Next problem.
The problem was that the front end developers involved decided to use Next.js to replace the front end of a mostly complete Django site. I think it was very much a case of someone just wanting to use what they knew regardless of whether it was a good fit - the "when all you have is a hammer, everything looks like a nail" effect.
It is Vercel's "fault" because it sold Next.JS as a BFF (Backend-for-frontend), a concept that didn't exist but helped them sell lots of hosting for React, something absolutely unexpected to every old school React developer.
I did a search and a lot of people are promoting the concept. Maybe it makes sense if you have a strong reason to use micro services, but for the vast majority of systems it seems crazy!
The problem was that the front end developers involved decided to use Next.js to replace the front end of a mostly complete Django site. I think it was very much a case of someone just wanting to use what they knew regardless of whether it was a good fit - the "when all you have is a hammer, everything looks like a nail" effect.