It rather depends what is being conserved - they may not like plastic windows, but if the preserved town hall was subsiding they would probably accept modern underpinning methods to save it.


Similarly restorers of old radio and TV equipment go to varying lengths - some will hand build waxed paper capacitors and so forth, while others prefer to fit a modern  polypropylene capacitor inside the jacket of the old one. But no one rebuilds anything with the polychlorinated oils of yesteryear.

I suspect that your old car may run on new oil, and asbestos free brake pads.

In a similar way we may like gleaming brass lamp holders, but not the original rubber covered flex and lack of earthing.  To be done well a case-by-case understanding of how the system used to work, what the dangers are of retaining it, and what the alternatives are, all need to be agreed. One for a bit of engineering judgement, rather than the usual hammer the screws in and run away merchants. 

Also we do not always want to preserve very much, as the build quality from some periods were better than others. One or two houses representative of the Victorian slums is quite enough, the rest can be demolished or re-furbished, nor do we all want a 1970s car I suspect...