Earlier quoted context omitted.
There are many projects where just doing that would be the equivalent of a full time position. Even saying no requires a skilled review.
It would be nice if you could turn off accepting pull requests whilst keeping the project alive.
Why does GitHub persist? It's not like forcing the feature enabled helps anyone. The maintainers will still not merge if they don't want to. There are bots in the marketplace that will close PRs with a message. GitLab has a toggle. Why is this so important to GitHub?