There's no reason why a company would be required to release the source code once copyright expires. Someone finding an old copy of the source could spread it around, but that's about all.
However, with copyright expiring 70 years after the death of the author(s), by the time copyright does expire, the sources will have disappeared.
Under the Sonny Bono Act in the US corporate copyrights were extended beyond 100 years. The original US Copyright Act was limited to 14 years, and allowed a single renewal of the same length. After that, public domain. The short term goal was to provide authors with some income during the term, but with the ultimate purpose of enriching the commons to benefit the public. "To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries." US CONST, Art I, Sec 8, Cl 8. It could be argued that copyright copyright terms of over a century, especially during a particularly dynamic period in world history such as ours, no longer serve that purpose. As you point out, the end result of the current regime is the debasement of any real practical value many works would otherwise have. All that will be left are the cold entries in whatever royalty ledgers survive.
In my opinion, copyright and patent law needs to be altered significantly, with the copyright term obliterated so people can build upon advances in technology and culture within a reasonable amount of time.
Sadly, I don't think it's realistic to think copyright terms will ever be reduced. They'll only be extended, probably when one of the many Disney properties is about to become public domain.
However, with copyright expiring 70 years after the death of the author(s), by the time copyright does expire, the sources will have disappeared.