[TeMPOraL]

The companies will say that they're still using parts of the old code in newer products covered by copyright, so this won't happen.

Besides, we'd have to have copyright periods on software of 3-5 years, 10 tops, for software copyright to even make sense. Not 70+, which is longer than any recognizable computer industry ever existed.

[MereInterest]

I am assuming that this would be part of a wider reform of copyright. For the case you mentioned, already under current copyright law, creation of a derivative work does not prevent a work from entering into the public domain (albeit after an unconscionably long period of time). To prevent such a argument being made after the fact, source code should be placed in escrow at the time of publication, with copyright protection given only after it has been verified that the provided source code can reproduce the binary being protected.

This arrangement would also protect the public good in cases where the original company has gone bankrupt, or where the source code would otherwise have been lost.