One immediate problem that comes to mind is that you need a smartphone to take public transit. So if there's a teen without a smartphone, they can't take the bus, nor can someone who's phone died, etc.
One of the amazing things of the current system, as simple as it is, is that it's predictable and doesn't require coordination. You can walk to a bus stop and know that a bus will arrive and take you where you expect to go, same as the last time you've taken it and the time before that. You don't need to look up a map to see what today's route is, or to see where the stop is, or to let the bus know you're waiting for you. You just show up at the bus stop and the rest just happens in a predictable and reliable fashion.