Not sure reality agrees here. Unless the project is huge, it will most likely not see any money. With copy-left code they are at least required to share their improvements and contribute that way. (I am aware of the no-gpl policies of many companies)

What's probably a better Model are Nextcloud's "SupportaaS", Sourcehut's consultancy, or mailcow's SaaS. (I also see SaaS critically, but if it's Libre I'm okay with it)