It would affect the standard library of rust, not rust itself I think. Second, the word-size matching the architecture has only a slight performance impact, i.e. operations on a 64-bit word on a 32-bit architecture take more cycles/instructions than 64-bit words on a 64-bit architecture.

It's only important for things like pointers that they match the size of the addressing space, and not even that is a very hard constraint, just a very convenient one.

16 bit architectures are weird enough that a lot of tools don't support them. And 8 bit architectures are basically only used for things like Arduino nowadays.

However, using a 32 bit number to store values that will never be larger than 16 bits isn't that bad, it's just very slow.