In case someone from backblaze read this :)
Thats because a "worn out" flash sector is never fully worn out - it can still store some data, just less than the error correction can correct. It is possible to combine two sectors to have extra error correction data to still recover a sector. Now you have less than half the performance.
Worn out flash also doesn't hold data long - perhaps only a few hours before too many bits have flipped and it is unreadable. To fix that, you need to rewrite the data, which slows everything down more.
And now that you have a bunch of unreliable sectors, you also need "super-sectors" which can do sector-based hierarchical erasure coding to recover data from sectors where even the methods above have caused data to be lost. This slows down writes even more.
In the worst case, reading a single sector requires reading every sector on the drive to reconstruct. Clearly thats going to be slow enough the drive will have stopped being used long before that.
Sadly some drive firmware doesn't implement some or all of the above, so they appear to have "failed" and become unreadable, which IMO is inexcusable when it's very easy to design so that worn out drives become slow instead.
SSD firmware is also a spectrum between "correct" and "performance", and I know of no SSD's that for example maintain all of the acknowledged data on a power failure. Sure many SSD's may typically do that, but that isn't a guarantee when the power fails in worst-case conditions.
Old Apple SSD's for example have a special extra wire on the connector specifically for "impending power failure" to help them do that. PC SSD's don't even have a standard message to mean "power failure expected in 250 milliseconds".
Do you have any examples or references that drives that implement your suggested algorithms?
I wouldn't expect drives to have sector-splicing and super-sectors (though multi-level cells regularly store less bits/cell), infinitely degrade their size via write-failures or rewrite data every hour. Especially frequent rewriting would self-destruct the drive if it weren't already preceded by catastrophic data loss.