Systems are built to be fault tolerant because hardware errors are inevitable. Bit-flips in RAM and disks, line noise in transmission circuits, voltage fluctuat

Systems are built to be fault tolerant because hardware errors are inevitable. Bit-flips in RAM and disks, line noise in transmission circuits, voltage fluctuations, etc. And these problems are statistical. In a new machine the chance of error may be 1%, then as it ages the error rate increases to 10%. Repeat something a billion times and those percentages mean you have a million errors. Not only in time but density; a chip with a thousand transistors is going to have fewer problems than one with 10 million. That's why a much older system can be faster than an old but more recent one. But also because the older system will be less fault tolerant. Back then the cost of handling a bit-flip or low-voltage wasn't higher so they ignored the errors and continued to run at full speed as it corrupted your data.