The fact of being open source means if your contributions are rejected, you simply need to fork. The open market competition will soon determine if your contributions were truly valuable and whether the original project's owner misjudged, or that contribution was essentially worthless.
Being able to market your work and build community are key skills for open source contribution. The history of forges is littered with interesting code that never got enough attention to go anywhere.
No project is obliged to accept your contributions. It's on you to market them, either to the maintainers of the existing project for merging, or to the community through a fork.
It's not a key skill at all, as not everyone is interested in popularity.
Then marketing it to convince others to even try... If the idea is too niche it might never hit that threshold for uptake.