Depends on a few factors, AFAIK as a non-lawyer. If the license allows closed-source derivatives (i.e. is permissive rather than copyleft), then anyone can create a closed-source version with all of the contributors' changes, including the original maintainer. And anyone can choose to keep it open-source. The community contributions still to some extent belong to the contributors, though the license waives most of their rights.

Some projects are copyleft, but contributors are required to sign a license agreement (a CLA) which allows a single entity to change the license as they desire, including to closed-source - this is a good reason to avoid such projects. The contributors don't own their work in such a case, but they can still fork the old project as it was before being taken closed source.

In a copyleft (e.g. GPL) project with no CLA, it's illegal for anyone to make a closed-source version, and a major contributor could sue even the maintainer for doing so.

In all such cases, the change to a closed-source model does not erase the existence of the open-source code with community contributions. A fork is always possible.

Quick comment about posting to lemmy: the first line (when followed by a blank line) is taken as the title of the post, so things work much better if you put your tags at the end or in the body and, if you can, write a first line that can work as some sort of title. With this post, we’ve got the tags and their URLs in the title.

Some projects require contributors to assign ownership of their contributions to some legal entity (e.g. a company).

In that case, those contributors no longer have any control. It's all up to whoever the legal owner is.

However, many projects merely invite contributors to license their contributions to the project under the same terms the project already was under.

The Linux kernel is an example of this. It was released by Linus Torvalds under the GPLv2, and contributors added their own changes under the GPLv2, without assigning ownership to Linus. Linus only has permission to use those changes under the terms of the GPLv2. They don't belong to him; he's only using them in the Linux kernel under license from their creators.

So, there is nobody who can "take Linux private", because doing so would require the assent of lots of contributors; many of whom would be unwilling to go along with such a scheme.