One could also argue that Torchwood was an "Artifact Title" by the time of Miracle Day, since Torchwood had ceased to exist as an organization by that time. So it's not unprecedented even within this franchise.
The reason there are so many "Artifact Titles" is that titles aren't really about conveying accurate information about content, they're about promotion and name recognition. Keeping the same title is a way of saying that it's the same series, that even with a cast change, the viewer will still be getting basically the same experience. Changing the title would mean having to start over from scratch in the name-recognition game.
You don't think there's a slight
difference in having a series named "Torchwood" where Torchwood the fictional organization no longer exists... and a series named "The Sarah Jane Adventures" when the actress playing Sarah Jane has passed away and everyone knows she's passed away? Just for name recognition?? One isn't in bad taste; the other is.