Those are necessary but not sufficient conditions to having a healthy community which actually continues developing such forks, likely for free if the main company backing the original product goes away. I don't know why people think OSS is a panacea to community development, lots of things are OSS and have few to no contributors.
Honestly even the previous stable edition of Next.js before server components would be fine to peg to.
But history has shown that when a framework or backend was sufficiently popular that people did indeed step in to support it. It happened with Node.js itself (which Next is built on) a while back with what was then called io.js, before io.js eventually re-merged with Node.js after the organizational kinks were worked out.
Next.js promotes serverless deployments and in that scenario it’s not sufficiently Open Source. Vercel (or amplify) adds proprietary stuff that’s missing in next itself. There is an Open Source serverless implementation for aws lambda (open next), but it’s not official.