There is one thing I think will be problematic with at least some software becoming public domain and that is when the programs uses technology that originally was licenced from third party developers who may still have the copyright to their stuff.
One good example of this is Personal Paint, because when Cloanto decided to make their program free, they had to remove the GIF save support first since it was licensed.
In Cloantos case this was not a major issue, but with software where fundamental parts originally were licensed from third parties, it may not be as easy to make them public domain, and opensourcing them might even be worse.
