Do you really think that will happen? Looking at Activity Monitor in my desktop, only about 50% of the processes are 64-bit, and many of those are ones associated with the OS. Of the ones that are third-party applications, many still have a 32-bit executable I could launch if I wanted to (do a "Get Info" and see if there's a checkbox)--or needed to, which is why Safari, for example, still gives you the option.
Since the benefit to 64-bit apps on mobile seems less clear than it does on the desktop right now, I'd expect this transition to happen even more slowly. This is in addition to the fact that the 5s is currently their only 64-bit-capable product, whereas Macs have been 64-bit since the Core 2 Duo (2008 or so?). For comparison, iDevices from that age are ancient now and, for example, don't support the latest OS. I expect the same will be the case with today's devices when (if?) 64-bit becomes the norm on mobile.