CRC can't save you from faulty RAM. It can save you from bitrot in data at rest and from transmission errors. If you have faulty RAM, all bets are off. The data could be corrupted after it's been processed by the CPU (to compute the CRC) and before it's been sent to the storage device.
Arguably, the real reason CRC is useless is that most people don't care about the data integrity of their PNGs. Those who do care probably already have a better system of error detection, or maybe even correction.
Arguably, the real reason CRC is useless is that most people don't care about the data integrity of their PNGs. Those who do care probably already have a better system of error detection, or maybe even correction.