Anything is only worth what some idiot will pay for it. I say idiot because every person has that one item that makes them go full stupid, and they become willing to spend way to much on it just to have it.
No matter if its the latest phone, old motorcycle, or supercar, everyone has their weakness. I got some flack for saying some CB wasnt worth over $1k to me, and no matter how rare that bike was, it simply would NEVER be worth anything over the $1k dollar mark to me, because its not my "stupid" item.
Your question regarding "are custom bike builders destroying historical artifacts?"
They are destroying them, just as you are by riding them, those original nos parts are wearing out with every mile, and every mile that ticks by, brings up your chance to lose your life or the bike in a split second.
If you want a historical artifact, you better seal that baby up in a bubble, and just look at it in your humidity controlled environment.