A well written SSD firmware simply slows down with age. It will never get to failure because the slowdown gets so extreme it becomes unusable. The drive also gets slightly smaller (by passing write failures to the OS to mark sectors bad).

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.