I don't know how it is with iOS, but my (Android) HTC One M9 has 32gb of basic storage (with the facility for adding an sd card). This seems to be mainly down to the fact that the OS and other pre-installed software takes up something like 12gb, which gives you precious little headroom on 16gb. The M8 had 16gb, but the OS overhead was only about 8gb
Assuming other manufacturers offerings are similar, then, yes, 32gb is an absolute minimum, and more would be at least desirable if there's no expansion option. The cynic in me says that no expansion = a premium price for the added storage (and that's not confined to Apple).
