People get funneled to their paid plans out of necessity. While NextJS is open source, the back-end to run it is not. That is where all the complexity lies. Even on Netlify, you run into crazy issues with things like their image stack. It does all this "Optimization" and caching that make it completely impossible to reason about and you run into implementation problems from the providers. Sure you can just run it in a container but then you are taking on all the complexity of NextJS preoptimizations without getting any of them.

Serverless framework attempted to make this stack to run yourself for Next but it is buggy due to the complexity of Next. Open source includes being able to run it. Releasing the framework and funding OSS that also enhances NextJS is nice, but it is a trap because if it comes time to seriously run it, your only option is vercel.