But still, even if someone had to buy another unit because one became kaput, they still bought another unit and Sony still sold another unit.
That's irrelevant. The numbers they print up during the lifetime of a system are used to show retailers how many potential game buyers there are out there. When in fact there are far less due to this very reason. There might be 100 million PS1's manufactured and shipped to stores, but I'll bet that less than a quarter of them still exist.
I thought the point was to see how many units they sold, not how many units are out there functioning.