I tend to replace my drives (if $$$ permits) every 2 to 3 years unless one of them fails before that. And seeing as they are willing to give 2 years of warranty on some of those renewed drives, I'm not really that concerned.
I am, however, buying 2 brand new 20TB drives for parity, which would be my biggest concern. On top of that, I have a mirror of my server in my brother's house over 2K miles away (beats paying cloud failover, for sure).