There are plenty of people switching machines that can be completely taken apart and replaced or upgraded component by component. Some other post around here mentioned eBay: plenty of completely modular HP and Dell laptops (and not even in bulk) for sale.

Most computer repair shops around here have disappeared because it's no longer the way people buy and use their computers. At least not enough people to keep those shops running.

Sometimes people upgrade because they feel like it. Doesn't always have to do with the numbers.