You're assuming that 4/5 of the laptops remained in pristine condition. I doubt that very much. You need to take account for the ordinary wear and tear compounded together with the impact of abusive 1/5 of users that do an excessive amount of damage (requiring a total overhaul.)

And anyway, if 1/5 of laptops needed major repairs and some of them got it, those go back into circulation. Are they still original laptops? (Ask my grandfather's axe...)