> But, and this is important, we were monitoring the systems for drive failures and replacing them within 24 hours. Ditto for power supplies. If you don't monitor your hardware for failure, redundancy doesn't mean anything.

It does still mean something.

If you have a 5% annual chance of failure and no redundancy, your five year failure chance is 23%.

If you have redundancy and literally never check for five years, your five year failure chance is 5%. That's already a huge improvement. If you do an inventory of broken parts twice a year, still no proper monitoring, it goes down to 0.6%

For 2% the numbers are: 10% 1% 0.1%

For 10% the numbers are: 41% 17% 2.6%

(The approximations for small percents are x*5, x²*25, and x²*2.5)