We don't have any data yet. The oldest Backblaze pods contain hard drives that are not quite 4 years old, so we haven't seen any old age mortality yet.
Here is another totally random thought: We pay $1,400 / month / cabinet in physical space rental plus electricity, which comes to about $5 / drive / month / cabinet. Even if the old (smaller) drives last forever, there will come a moment where it is just a good financial decision to copy all the data off of them onto denser drives because in some number of months the savings in physical space rental and saving electricity (assuming energy use is fixed per drive) pays for the new drives. If a new hard drive is 10 times as dense, it saves Backblaze $4.50 / drive / month in physical space and electricity rental. In 22 months it pays for the $100 replacement drive. (I did that math super quick, so let me know if I'm off by a factor of 10.)