I have main sewer line that I guess is rotted out? It is cast iron. The house is pier and beam with a slab addition. The line runs under the addition. The plumber ran video on the sewage line and wants to replace it. He also wants to replace casr iron branch(drop) lines that connect to the main line without having looked at them. The house is 70 years old and says the age is what justifies the change? Does this make sense and what kind of cost does this look like?