I guess, since the SDXC standart supports upto 2TB, if the OS does support, 128GB might not be the limit. In my opinion this is the reason, old devices seem to support new 128GB cards.
But I have a different question.
What are the access / read / write speeds for the internal (16GB) memory the Z2 has?
I assume, they must be superior to those of external SD storage.
Using an external storage for applications is never a good idea in my opinion. Dismounts, corruption, file system preference, etc...
Well, pictures, video, documents, no problem.
Morover, it's against the philosopy of posix to use a mounted external storage for system assets (which I tend to include applications within).
Yes, of course I'll ask for more

Cause 16GB is nothing today...
External storage is slow, non-native, mounted...
So, how come? Why did they not provide 32GB like others do?
Is it going to be on another model later?
This, at the moment is the sole reason I'm not buying the Z2 (and also my doubts on the glass surface and the screen endurance may too have an influence on my decision)...
By the way, can anyone enlighten me on the speed rates of the internal storage of z2?