For many Joe, without parts machines to supply the parts, it would be cost prohibitive to order and replace parts when you can buy a complete machine of the same model for less money. I understand that you like to tinker with the machines and that's ok...just as it's ok to part out machines that are common and easy to come by.
There's nothing better to learn on than a machine that you can't hurt by making a mistake!
S'all good, and live and let live kinda thing, cuz I seriously doubt that I'm going to change your mind, anymore than you could change mine!