When a hard disk is aged and near death, I've experienced unusual noises or intermittent operating system booting or utter failure to boot. In my experience when a hard disk is having problems death is imminent so I think it's prudent to replace a hard disk after about 3 or 4 years. I would backup a hard disk a few times a week to a Flash drive selected from a set that is cycled because I like having multiple backups because if there's a problem with the one that's 12 hours old on one physical device, I can use the one that's 36 hours old on another physical device. Since I believe Flash drives have firmware, it feels risky, so I have multiple Flash drives that I cycle through. I don't backup programs so my backups always fit on a Flash drive.
I haven't tried continuous backups but I would probably still replace a hard disk after 3 years even if I had a continuous backup. I kind of dislike continuous backups because I would probably have only one external hard drive to do it so that seems risky. Furthermore backing up a rotating disk with another rotating disk also seems risky.
What is it like when an SSD is old or near death and if that SSD is in my main system what is the prudent plan to deal with it?