I guess I’m not seeing what your seeing. I can’t tell that they installed a new deck over an existing one. Grinding out the rust and filling it with bondo or pool putty epoxy is an easy fix. Hopefully the OP has polymer walls. Easy enough to find out just take a magnet and see if it sticks. They were using polymer walls during that time period as my pool was constructed in the 70’s and we were lucky enough that it was constructed out of polymer. We converted it to salt water and I’m not sure we would have gone that route if it had galvanized steel walls. How bad can the corrosion get with steel walls and what gauge of steel is typically used?