Think of the classic "bathtub" curve (which says that young drives fail more frequently, old drives fail more frequently, and mid-age drives are most reliable).

That curve doesn't seem to match the data here. Or if it does, it says the "old" increase in failure rate happens at over 5 years.

I would guess backblaze will replace these old drives because they are too small/too slow/use too much power before they replace them for being too unreliable.