max_align_t is a bit of a kitchen sink and will specify larger and larger alignment in the future, e.g. 32-byte alignment for a complex _Float128 type (a pair of two _Float128 variables).  The alignment is also not generally useful for allocations whose size is smaller than the alignment.  (Many mallocs do not follow the C standard and do not provide 16-byte alignment when 8 bytes are allocated, although alignof(max_align_t) is 16.)