I think you have some fundamental misunderstandings here.
First off, there absolutely is an amount of "hot spares and RAID" that would save you from "failure rates that high", and ultimately hot spares aren't going to help anyway -- either you're actively running things in which case you need enough hot spares to handle disk replacements until the next time someone opens up the computer in question to add more disks, or at the opposite extreme you need enough active redundancy to survive a random 1/3 of disks failing.
With that said, it sounds a lot like you think the model here is "a bunch of drives sitting on a shelf", which isn't how anyone sane would run this type of service. The Backblaze model (at least as of 2016 when they last published POD designs) has all disks actively connected to computers. Those 2016 designs heavily use expanders which means that you're not going to get full bandwidth to all disks at once, but it's not like the disks are powered down and inaccessible. It almost certainly makes sense to use spare disk I/O bandwidth to run some frequency of scrubbing of existing data -- if only else to figure out quickly when disks fail so you don't need to keep so much redundancy around. Like, think of this as weeks to months between scrubs that verify that all backed up data is healthy, not accessing cold data after disks sat on a shelf for years.