"The copyright will last something like 70 years after the death"
For now. Then, before it is elevated into the public domain, it'll be
retroactively extended to 95 years after death. Then 120. Then 145.... And
effectively never be elevated into the public domain.
"The source code has never been released/copied (hence it is not copyrighted)
and will never be unless their authors decide so."
On the contrary: In countries that have implemented Berne (and most countries
of the world have - see https://en.wikipedia.org/wiki/Berne_Convention for a
map), copyright is automatic. See the note that "the Berne Convention
formally mandated several aspects of modern copyright law; it introduced the
concept that a copyright exists the moment a work is 'fixed'".
Not that it's talking of the work being "fixed", not the work being
"published."
An example of being "fixed" is the source code being saved inside files on
the company's computers. It's copyrighted the moment that happens.
There's a good chart here from Cornell showing the copyright duration of
"unpublished works" - http://copyright.cornell.edu/resources/publicdomain.cfm
While this chart is U.S.-specific, every country that implements Berne (which
as you can see is most of the world) would need to have some implementation
of the notion that the source code would be copyrighted at the moment is is
"fixed."
