[dredmorbius]

In a talent acquisition, disposition of source code assets would be up for negotiation among principals prior to closing the deal.

No reason founders couldn't specify this as a condition. Or make it an explicit term of their service agreement to customers/users well in advance of this eventuality. Something like:

"In the event COMPANY concludes business operations or discontinues SERVICE for any reason, including but not limited to sale or acquisition of COMPANY or relevant assets, COMPANY will license SOFTWARE under the terms of $FREE_SOFTWARE _LICENSE, in the preferred form of SOFTWARE for making modifications to it, inclusive of any build, deployment, operations, management, and documentation support."

Where the free software license would be specified, though generally I'd expect this would include a license meeting the FSF's Free Software Definition or the OSI's Open Source Definition and certification.