“Why would someone keep a passport that has been expired for 20 years?”
I just now came across your post...
Well, I can not imagine ANYONE destroying their old passports.
They are a perfect record of where and when you have been in the world.
My original passport is a rich treasure of the many countries that I have visited, and was once so full that I had to have
pages A-X added. I have since started a new passport, but I frequently look up data in my previous passport.
Why on earth would you shred it, unless you are a criminal?
My old passports are of no use to me and I can pretty much remember most of the countries that I have been to.
And no, your passport does not provide a perfect record of where you have been in the world since not all countries have a process by which to check your passport when you enter their country and in some cases it is a good idea if your passport is not stamped by a country that you visit.