I agree up to a point with that, but I seem to have been making some dangerous assumptions. Firstly, I thought he was talking about metal conduit (which we now know is incorrect). Secondly, I thought he was talking about an 'unplastered' stone wall (which I now realise is possibly also not correct), in which case, regardless of age of the wall, I don't think that white plastic would look very nice on it! I have both unplastered brick and unplastered stone in my house, and I personally wouldn't want to see white conduit or mini-trunking on either - but I suppose that's a personal thing!400-year-old cottages did not, when built, have anything electrical, so any modern additions will by definition be out of place. So whatever goes in needs to combine practicality with whatever aesthetics suit the rest of the decor and/or the owner's preferences.
It's interesting what Bernard said about putting copper pipes inside white plastic conduit. I have been known, to satisfy my own view of aesthetics, to do almost the opposite - putting cables inside earthed copper pipe!
Kind Regards, John