Anyone could fork Wordpress and a whole suite of useful addons any time if they felt qualified. But then what they would have is the responsibility to not only maintain a fork of fucking garbage Wordpress, and also a bunch of other software, but also continue to make it viable without wordpress.com. And what's your prize? A lot of people expecting to migrate from Wordpress and wanting to pay you nothing.
People who want to customize an OSS CMS are using Drupal. Most people using Wordpress don't want to learn anything. I don't blame them, when I chose Drupal over Wordpress (and some other options) I wasn't really trying to learn php, and I'm happy to report that I really never did. I have occasionally learned just enough to contribute some minor patches — code that is long gone because Drupal is now Symfony-based and the way you do basically everything has changed since I knew how to do anything meaningful, back in version 7 or so. (I started with 4 but quickly dropped that and went to prerelease 5 as it was the future at the time.)
TL;DR: Without that corporation you don't really have Wordpress anyway, and for those people there's Drupal which is technically superior FWIW