You might be able to sell them on this by mentioning insurers reduce premiums on rewired buildings!
This might get them over the line but the simple fact that they have a legally enforceable duty to maintain and repair common property should be enough. They know this, you’d think, which is why they are arguing that it’s lot property.
You just have to tell them they’re wrong and they can either waste money on lawyers and then fix it anyway, or just fix it.