I suspect this is more of Micron's 10-20 year commitment thing, where they guarantee a *sub-set* of parts will be supported in a form-fit-function sense as die are shrunk,etc.
Price may be a wash or better, consider also volumes (used in multiple BOMs, or bought at fire-sale pricing), and stock-room shelves/purchasing.
It's quite tight to the F746, might route optimally.
32-bit would have better bandwidth and space, so wasting that. But STM32 has limited pins and there's probably packaging/cost issues, along with PCB cost/complexity.
Design is a multi-variable problem, and sometimes flexibility comes from different places.
