[Kye]

There's no practical way to make WordPress closed source. 23 years of contributors would have to sign their code over. It might have been possible to get enough people to do it and rewrite the rest before the community was put in such a polarized state, but now there will be significant contributions made by people who won't do it on principle.

[emn13]

That's good to hear; some projects require copyright assignment before merging contributions, or some extended license that might permit re-licensing, and I wasn't aware of the status of the Wordpress code base in that respect.