2001 was when Windows XP was released. That's at least a few "eras" in computing ago.
Any open source project is only as good as it is today not what it was.
If you like to test it, I would be very interested in the result of trying to get linux running with only code written from the last 3 years. I would assume that almost a majority of the code would be there, but then I would also assume that most of the essential core is basically untouched except for security patches. It would make for a very interesting read and a HN worthy article.
Have kernel images doubled in the last years?
Maybe 10 years will be little more accurate?
Well, now looks little different...