Anyone who regularly uses public transit in the USA could tell you this isn't a serious solution to a real issue.
You use either Google Maps, Apple Maps, or the Transit app to tell you what to get on. You either buy a local nfc card or have an app on your phone you tap on the sensor to pay.
Having to use more than one app isn't an significant burden compared to the fact that that trains and busses don't come to the places I want at the times I want reliably.
Some cities come close to that, but most don't. Ignoring the fact that most cities in the US don't have expansive transit, even _getting a transit card_ can be a challenge in many cities.
* in Denver, CO - there is a custom app that works fairly well until your phone is dead. Couldn't find a place to purchase tap cards, but the paper ticket vending machines worked well enough.
* Seattle, WA - still working on mobile NFC tickets, and the current mobile ticketing system seems to only allow existing (> 1 year ago) customers to pay using a credit card. All new customers seem to only be allowed to use the app by filling up their balance using cash at a convenience store. Tap cards were an extremely rare find last summer. Only a select few buses seem to send their tracking data to Apple/Google maps, the OneBusAway app was really the only one that was close to accurate.
* Dallas, TX - yet another custom app implementation
* Austin, TX - _another_ custom app
* Portland, OR - can only buy tap cards at some retailers, BUT they accept contactless credit cards and have an app that integrates with mobile NFC
While I haven't been to NYC and a number of other cities with large transit systems, Portland OR and Washington DC had the best implementations, largely because they simply integrated with the native mobile NFC wallet. DC even lets you provision your card straight from the iOS Wallet app, though you can't use a standard contactless credit card.
IMO the solution isn't an app, but rather simplifying fares and accepting contactless credit cards for fares. In Portland you "earn" your passes by riding rather than needing to purchase in advance - once you have paid $100 in a month you're automatically exempted from fares for the rest of a month.
I'm quite familiar with the situation in Denver. Your criticism is that it doesn't work well if your phone dies. True, but I'm replying to someone suggesting "Uber for Public Transit". I presume that involves a app rather than a physical product.
It would be kinda neat if we standardized across the country on one app, but it's nowhere near the top of the list of things to address. Most people mostly use one or a small number of public transit systems. You can just have more than one app. Municipalities are also increasingly opening up the payment system such that apps like Google Maps and Transit are able to offer buy ticket buttons.
You use either Google Maps, Apple Maps, or the Transit app to tell you what to get on. You either buy a local nfc card or have an app on your phone you tap on the sensor to pay.
Having to use more than one app isn't an significant burden compared to the fact that that trains and busses don't come to the places I want at the times I want reliably.