Assuming a 14 year copyright that would mean gcc 3.0 enters the public domain but the current version obviously has it's own duration. I don't see why they should get special treatment. Do they really need copyright of 14 year old versions of software and monopolise it? Are they that scared that someone takes some old version of gcc and uses it for a commercial application? Also nothing stops you from taking a public domain work (even your own) and slapping a new license on top of it if all you care about is the license not the monopoly.

Also the way the current copyright system works means those free software licenses are already "dead" by your definition. It's just through pure chance that the duration keeps getting extended retroactively. The only difference between the current system and the proposed system is in the duration and the fact that in the old system all works had effectively the same duration again through pure chance because it's based on the lifetime of the author.