if someone is worried about not being able to get the headspace set correctly when they rebarrel a rifle.....well then, my point stands about people who know their craft. It's obvious you are just trying to be disagreeable, and that's fine. You are welcome to spend your time arguing about who is right on the internet if you don't have any customer's rifles to build.
I've been through the trial and error of seeing who builds fine rifles and who is a basement bubba. And to be fair, I'd have to see your work to know which one you are. But after years of seeing crap work, I have become very careful about who I take rifles to, and since this is a point which has come up before in talking with them, I am saying that the 2 best gunsmiths I know both take the barrel off when they thread. And they also reinstall it to the proper headspace.
that's all I really care to argue about the topic.