A number of years ago, well before smartphones were a thing, I spoke with a PRT designer who was planning to make smartcards the preferred means for users to access the system. At the time I counseled him that a public system required anyone be able to pay a fare in cash -- what if the smartcard was lost, or damaged, or forgotten at home or office? What about members of the public who don't have a smartcard, for whatever reason? Tourists for example.
These reasons and others also apply to the idea of making the smartphone the centerpiece of transit access.
What if someone has lost or forgotten their smartphone, or doesn't own one? What if they're out of minutes? What if they are a senior citizen who has a no-frills non-smart mobile? Or what if the battery is simply dead?
And what if people are trying to access the PRT system ahead of, during, or in the aftermath of a wide scale public emergency, and the cellular network is overloaded or not working?
These reasons are why, despite it being possible to completely automate stations, at least a significant number of them, if not a majority, should have flesh and blood customer service staff.
Which reminds me that in King County the Metro buses are designated Safe Places for youth in trouble. Completely automated systems should have policies that recognize and facilitate these and other public priorities.
I know it might be more streamlined or efficient (at least for transit agencies) if everything could be done by phone. And it might be cheaper (for transit agencies) -- I've heard that cash-handling is expensive (although it's probably banks that said it).
But we're talking about public transit, the first priority is provision of service, not making things convenient for the operator. You accept the coin of the realm or get out.
(The following has been put in Bold so that commenter Mr. Reductio et al will notice it)
Future users of advanced automated transit should expect a level of service better than what they get with online banking. Or their wireless provider.