It doesn't have to. The content could remain out in the open.
Although you are correct in that I am suggesting a paywall there are other ways this could work:
- Subscribers could set MDN project priorities in terms of which technologies get focus, topic expansion, expanded example code, etc.
- Early access to incomplete documentation could be given to subscribers.
- Issues with documentation or example code coming from subscribers could get priority treatment.
- Offline versions of the documentation could be subscriber exclusive.
The bottom line is that a tragedy of the commons situation needs to be avoided. Is a paywall really worse than letting bit-rot settle in on the existing MDN content?