I must remember that copper only lasts 40 - 50 years as someone forgot to tell that to the roofing contractors who over the past 1,800 years used copper flashing exposed to the elements acid rain, snow, heat, ice and acid bird droppings yet these old buildings seem not to know copper lasts only 50 years.
If the copper was installed properly and the Ph of the water is non corrosive there is no reason why it cannot last a long, long time.
how about the dummies who went for STATE OF THE ART plastic under slab heating systems and then the plastic piping failures popped up all over the country with lots of finger pointing as the installers would never take the blame for not doing their home work.
Install and get in and get out as fast as possible is the plastic game in plumbing and forget about the failures and toxicity of the products.
Copper even thin wall such as DWV (drainage waste and vent) can be easily repaired (soldered) or relined.
If the installer suggested replacing the copper with cast iron I may be inclined to agree but to use and inferior product such as plastic waste I would seek out another opinion.
Today copper prices are sky rocketing and many unscrupulous contractors suggest removing copper and using another product and sell the copper as scrap metal.
I recently heard of a roofing contractor remove perfectly good copper counter flashing and gutters and leader piping as the metal scrap yard was offering a small fortune for this metal.
Personally even for drainage I use "L" tubing as the slightly thicker wall gives longevity to the system when wall space will not allow for cast iron
Want to kill a great copper system or cast iron system? then use chemicals for drain cleaning that is a plumbers dream come true.
Chemicals are a plumbers best friend