Theory - A
Logically Apple has never cheaped out on parts, this year the demand is too high and they've dropped Samsung out of the picture completely for SoCs and NANDs.
Hynix and Toshiba can't produce the amount they need, so they've included SanDisk as one of their supply chains for the first time and in the past, I've never seen them on their list for anything.
The SanDisk TLC aren't performing well and we have our saga today.
Theory - B
It's all about the cash flow for Tim Cook, pleasing the customers and giving them what they want. In the backend, figure ways to reduce the cost for higher profits.