[toast0]

> pipes for those will last 100 years, easily

My wife served on a sanitary district board. Decent underground sewer mains have an expected service life of 70 years. Some will last longer, some won't hit the expectation. It depends on site conditions, materials, etc, etc. Many will probably last to 100 years, but I wouldn't expect it to be easy. Especially if you had any of the not decent materials (Orangeburg pipe was common in some areas and is basically wood pulp/fibers mixed with hot tar; service life could vary between 10 and 50 years; the major manufacturer went out of business in 1974, as PVC and ABS pipes rapidly replaced Orangeburg in the materials markerplace).

Actually there was a huge nationwide boom in sewer building post WWII, especially in the 50s, and you can expect that infrastructure to need some largescale replacement over the next 20 years or so. Depending on system design and how housing and industry developed, some systems will be able to just inspect periodically and replace as needed, and some systems will probably take the opportunity to do a more modern redesign (older cities tend to have combined sanitary sewers and storm/runoff drainage; if you're tearing up all the streets to replace the sanitary sewer, it might be a good time to put in a parallel storm drain system)

I'm not sure about water mains, I'd guess they need more frequent replacement since they operate at pressure.