It doesn't have to be fair. Things come and go. To host information for X years, statistically the things most likely to last are those that already existed for at least that amount of time.

Or you can take your chances, which, again statistically speaking, is not good if we're talking about websites lasting 500 years.

No, the point is you can’t use the fact that websites are currently under 500 years old as evidence that they won’t ever be 500 years old.

True, but what percentage of electronic records from the 70s are still readable?

Sorry, that was the point I was trying to make, perhaps I should have made it more explicitly.