Keep or replace old cast iron under slab?

I'm in the process of moving a drain in the basement of my 42-year-old home where the drain plumbing is under a slab. The pipe I was working on (a 2" shower drain line) was completely corroded through, so I started excavating. Now I've exposed the entire system near the project site, and its pretty much the guts of the drain system in my home.

Two Questions:
1. Should I replace everything since I have it open?
2. Should I switch to plastic or stay with the no-hub cast iron?

Thanks for your input!

Here is the original 2" pipe that was corroded thru the pipe bottom




After seeing the corrosion in the picture above I decided to check everything else in the vicinity, including the toilet. I didn't find any moisture in the dirt, so I suspect its not leaking. But now I have the problem: Should I replace this stuff while I have it exposed, potentially opening a can of worms (and disabling the home plumbing until it gets fixed), or should I leave it?




The tee that connected to the original shower line shows severe corrosion, but its solid.




Here's another view

I continued excavating to inspect the bottoms of these lines. I found a hole on the bottom side of one of the fittings in the middle of the mess. If I would have left it alone, encased in tight-packed dirt, having sat there for 40 + years, would it really have mattered if the drain channel were formed by a rust/dirt path? It wasn't leaking until I excavated and probed the bottom of the pipe. Now I have to rip the entire mess out.

Should I bite the bullet and get the main drain re-lined (trench less?) since I'm going to have to expose this area up to the 4" buried main drain line?

This project is an example of the trouble caused by unemployment ... too much time on my hands...

Homes built up to the mid-1960's generally used cast iron drain lines. Cast Iron rusts from the inside out and generally lasts from 50 to 70 years before needing to be replaced. However, any home over 35 years of age is at increased risk of a failure at some point in the drain system due to aging. When drain lines are visible in a crawl space, the first sign that the pipe is at or near the end of its life will be a build up of rust and waterborne chemicals that has deposited on the outside of the pipe at spots where the pipe has rusted through. Some moisture has come through each of these spots in the past even if they are dry at the time of inspection and it is only a mater of time before a larger leak develops. Consequently, it is the opinion of many professionals in the field that any sections of pipe that have rusted through need to be replaced, even if they are not actively leaking.