Jon, I'm double-posting my reply, in case your blog doesn't like links and images in replies.
A lot seems to hinge (or henge) on (a) how current-day people perceive the object in question, and (b) which profession has custody of it.
By way of a few examples:
1) The SS Great Britain in Bristol.
It might have been a complete wreck at one time, but it was perceived as a great piece of engineering, worth the effort of restoring to better condition. Marine architects and engineers had custody. It was restored.
2) The Antikythera Mechanism
Now recognised as a fully working mechanical computer built c. 200 BC. Archaeologists have custody, didn't know what it was, and got nowhere with it. But engineers have taken detailed X-rays of the mechanism using a technique called linear tomography. The original is just too far gone to restore, but working replicas have been made.
3) Silbury, Avebury and Stonehenge
Not generally recognised as purposeful engineering of any kind. Barely and grudgingly recognised as astro-science of any kind. English Heritage has custody. It's all a mystery to them.
I did once suggest to EH that Silbury should be restored to its original condition.
They must have been struck dumb with shock (or thought I was taking the piss) because I never got a reply. If you do poke them hard enough, they will produce stock phrases like "Splendid isolation" and "ritual objects". Intellectual tumbleweed.