Well, I keep quite some of my old periphals as well: monitor, printer, keyboard, mouse, external drive, modem.
CPU, motherboard, gfx, to me form the most obvious performance characteristic of a system. Replace them all, and what you deal with is a new system in my book. You may put it inside an old shell - nevertheless it is a new performer.
You change your coats - you look different, but you stay the same person. Give your coat to somebody else, put another person into your coat - and you deal with somebody who happens to look like wearing your old coat, but nevertheless is somebody totally different from yourself.
