There is a pretty common problem with DO NOT FEED FEEDERS AT ALL, YOUR PIRANHAS WILL DIE
Thats pretty wrong. I know someone who has been feeding his reds ONLY goldfish for 10+ years.
Your P's will not be as healthy, and it may shorten their life span a few years, but it certainly will not instantly kill them. The harm here would be disease. The argument is Piranhas are scavengers, and eat "diseased" meals in the wild. I have not seen any real credible evidence on either side of this.
A varied diet for a Piranha is best, for an adult I would feed once per week - market shrimp, pellets, and once a month live feeders.
I have read a few interesting studies regarding predatory fish health and hunting their food, conducted by zoos.
I would not jump to the conclusion that feeders are BAD for your fish, but I would be careful because goldfish can carry disease, and should not be their main diet.
OP: My P's never got ick, but that certainly does not mean they are immune.