Smartphones didn’t replace PDAs, most people didn’t have a PDA to replace. Smart phones replaced whole categories of devices and bundled them into one: cameras, camera phones, PDAs, MP3 players, watches, timers, alarm clocks etc.
People keep looking for “the next big thing” and it just isn’t coming. The smartphone was a singular event in the history of computing and it wont be replicated within our lifetimes. At most it will be unbundled again, unevenly (eg, smart watches, and glasses)