I'm in the process of replacing an entire bathroom - had to rip out to the floor joists due to the shower pan leaking, etc. The home was built in 1975 so the DWV is a no-hub cast iron system. My main soil stack is in close proximity to the toilet, so it could be replaced, relatively easily, if needed. When I removed the no-hub fittings around the shower and toilet drains, I noticed the rubber gaskets surrounding the fittings are dry and starting to crack, when removing them. The other fittings going to the main soil stack appear to be in tack with no leaks, etc.

My question: Should I replace the old cast iron system with plastic? Should I just replace the gaskets surrounding the existing fittings or would I be ok to leave well enough alone and not do anything? What is the average life of a cast iron DWV system? Any thoughts? Thanks for any input.