I hear ya. It's only original once. And yeah, when a bike is original it would be nice to see it kept that way instead of parted out.
Thing is, not many bikes stay original for long. Something gets replaced either because it wears out, something lighter/stronger is available, to get a better fit, or simply to bling it up. Sometimes that happens and the components are period correct, but sometimes it's a bike that has been owned and ridden for years and ended up upgraded to 9sp when the original 8sp stuff died, for example.
Not a lot of us thought to keep things original, especially in the boom times of the 90s when every man and his dog had a CNC machine and a purple anno dip at their disposal.
And a lot of high end bikes were sold as frames only and built up from there.
Am I disagreeing with you? No. Because all those factors make a rare or original spec bike even more special.
I think an original bike should be kept that way as much as possible, same with bikes where the parts may not be original but are period correct.
On the flip side, if stuff didn't get parted out then we'd have more trouble that elusive rear derailleur to finish a build.
Enough of my musings...
Grumps