Apple announced long ago that OS 10.14 should be released in late 2018 and that support for 32 bit will be dropped.
That in on itself is hardly a valid argument, if you ask me, since people won't necessarily be
using that particular system for a good while after its release, let alone straight away.
Just look at me for instance, here I am still running a perfectly adequate, hassle-free 10.6 for nearly 10 years and happy as a camper

Not that a 64 bit BBox wouldn't be nice, but that's for entirely different reasons than this.