>Clearly the Windows NT kernel is older than 25 years, and is still making money.

Not the one from 25 years ago.

>Seems like a million years ago I wrote some games. The source code is long gone. (Well it's on 5.25 floppy disks in my garage for 30 years, so functionally gone.)

Legal requirement would have ensured great care in preservation of said source.

>I'm sorry to say, but making laws for old software is basically pointless.

If it's pointless, then copyright should expire much earlier.