should i replace underground galvanized dwv

I am remodling my basement bathroom. When I removed the old fiberglass shower I found the drain pretty corroded. I dug out the old trap and found that it was threaded galvanized. I tried to reomve the trap but it snapped in half . I was able to screw off the broken trap clean up the threads and put a PVC threaded adapter and a new PVC trap in. I poured the first layer of mud for my base. Then I did some research and many people say not to use galvanized threaded pipe underground. My drain pipe did have some thinning on the bottom noticable especially at the end of the threads.

The question is, should I stop, dig up the galvanized and replace or leave it and hope for the best? I am concerned because I puchased some expensive tile and fixtures for this project

Galvanized pipe has a finite life expectency, and I think yours has reached that point. It's safe to say if some is bad, the rest isn't far behind. "Hoping for the best" will come back to haunt you in spades sooner or later, most likely sooner.

ugh,I did not want to break open concrete. What do you think is the best piping for underground waste lines?

PVC is the best.

is it true that some areas only allow cast iron underground?