Not an expert so much but if the old pipes are ferrous then heaven
knows how much water is able to get through, they can really get filled
with stuff, to the point of being useless. My experience applies more
to the 1" supply (galvanized) and the 1.5" or 2" ferrous drain pipes, I
don't know if cast iron 4" stacks ever get rusty enough to stpo up,
that would be quite a thing to gaze into.

Personally I would rip out everything involved and do plastic, it's
relatively cheap compared to the ongoing hassle.

Also make sure you're bored out, out to the sewer, do you have problem
with roots, etc.