Easy. It would be the operator (me). I'm the only component that cannot be replaced without the entire system collapsing!
All kidding aside, in 40 years I've replaced every component many times over. There hasn't been one I regret selling as I've always enjoyed newer and (usually) better components, and a few vintage ones.
For me, the journey and enjoyment of a wide variety of components has been central to my love of high quality audio. Fun question though. It instigated quite the trip down memory road!