If the goal is performance and longevity, I can see how multiple chips might help with performance. But what about longevity? It seems that more chips means more things that can break, and when one of them breaks your whole hard drive will be broken. I guess the question is what's more likely, one of one chips breaking, or one of two chips breaking? Given what you said about read/write "spreading over" chips, maybe it's not so simple as assuming more chips equates to higher chance of failure.
More NAND dies means a given number of TB of writes will require doing fewer write/erase cycles per memory cell, because you have more memory cells.
NAND flash almost never fails a whole die at a time. An individual NAND die fresh out of the fab will already have a few defective memory cells, and as the drive is used, more write cycles will result in more memory cells failing and being retired. This gradual, partial failure is fundamental to how SSDs manage flash memory.
> It seems that more chips means more things that can break
Nope.
> I guess the question is what's more likely, one of one chips breaking, or one of two chips breaking?
Doesn't matter. In each case you lose your data. And if the system is so frail what addition of another chip rises the failure rate through the roof then the talk about the reliability of such system, no matter how much chips it have, is moot.
