"For example, FoundationDB was bought by apple and the product was shuttered. Under a living will, the product would become open source, as it is no longer developed by the company."

Would it? I assume the tech is still being developed. It's just that the continuing development is not offered as a product.

Further, once someone buys the company, don't they gain control over that company's assets? So if they no longer want to offer a product for sale, why should they?