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If you look at the history of desktop/server operating systems, it followed the same pattern as clearly explained in the article: problems start before 4GB and affect programming decisions as soon as users start working with files which aren't trivially small. Once 64-bit became the default, many small optimizations or larger design changes became easy enough that they actually happened and produced small but meaningful improvements

This specific case might only apply to Apple (I don't know, haven't read about it before) but the quirks of ARM's 32-bit instruction set surely apply to everyone else.

The transition could have happen later, maybe, with not too much repercussion, but why not make the jump now, especially when you have a whole new OS engineered to take advantage of it?

It will apply to competing platforms in about two years, when the first set of devices with 4 GB or more RAM start appearing.