"And every time you re-lay a road, you can retro-fit, until eventually everywhere will be connected (very few vehicle roads are more than 50-years-old in terms of the tarmac on them, I should imagine, even if the road has been there since the Roman ages)."
Sounds nice, but I was at a lecture by a civil engineer who was designing equipment to find all the stuff under the roads. First, some roads, particularly in London, are 'full', in the sense that with sewers, electricity, water mains, gas pipes, phone lines, cable TV, and more, there is no longer any room for digging underneath. Second, there are more or less no maps of this stuff. The recent pipes and cables have been mapped, but in most places in the UK, if you stand on a road and say 'where is the water main here?', the water company won't be able to tell you.
Retrofitting would mean that the water, electricity, sewerage, etc., would be cut off for over a week to somebody, probably much longer. I doubt you'd be impressed if someone said 'we are relaying your road. Move out for a week as you won't have any water or drainage'.