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?

Then again, in many orgs XP is barely being replaced with Windows 7...