OP, have you camera'd the sewer? As was mentioned above, rot will be from inside out. In your last pic, is it cast iron turning underground as well? Once it starts to collapse, it is just a matter of time before it all does. Iron pipe and water, or even worse when buried, ugh... Whoever thought that was a good idea is beyond me. And it is still spec'd today, although less prevalent. Once you start the chase, you will continue until you have ABS everywhere. It never gets better. Cast iron sewer sucks.
