My workplace's desktops and laptops, around 1,000 or so, still run WinXP Pro 32bit. Why? Users are comfortable with the OS. And, with our IT dept, server/hardware sub-dept, only having a total of 15 employees, it would take us months of training and retraining the 4,000 or so employees to use a new OS. (Especially since most of them are non technical workers that share access to computer terminals for inputting and copying reports, using software that only runs on WinXP 32 bit , etc.) As a result, the company downgrades all newly leased pcs from Win7 Pro 32 bit to WinXP.
At home, my mate and I have a total of 7 computers between us. 3 use Win7 Home Premium 64 bit (main gaming x2 and HTPCs), 2 use WinXP 32 bit (old gaming pcs, still in use because some games will not run on Win7... even in compatibility mode), and 2 others run Win98 (again, some games will not run in WinXP compatibility mode). Why the old PCs? WinXP mode for Win7 doesn't allow 3D acclerated games to run... thus the WinXP computers will be kept for quite awhile. And, the 2 Win98 computers are kept for a select few older games that either A. Run in native DOS and only native DOS or B. Require 3d acceleration from the old computers' Voodoo 2s to run correctly.
OSs, as well as old hardware, have their uses and shouldn't simply be thrown away when something new arrives. As previous posters have alluded to, the "if it isn't broke don't fix it" and "why upgrade if this still works" thoughts are not bad ones. Imagine going to a car dealership and BUYING (not leasing) a NEW car every 3 years. If everyone did that, then all used cars would end up just being scrapped.
The same is true with computer hardware and software.