Muni out in SF does this: basically the app tracks where you primarily use it, and if you fall within SF using primarily MUNI services, it gives you the paid tier for free. They also have custom prompts when you use Go, instead of asking if your train/bus is crowded etc, they ask if the stop is clean. I don't think theres a mechanism for them to show only Muni services, and I doubt the Transit folks would agree to that.
If Transit's limitations on the free version are too much, there's always CityMapper or Moovit.
If the MBTA put out an app, we'd all be on here complaining about how the UX sucks or how theirs some weird edge case that breaks things or how the trip planning is sub-optimal. Apps are expensive, both financially and resource wise, and sometimes it just makes sense to NOT do something especially in a resource constrained org.
Yeah, I mean I'm fine with the MBTA just providing data streams, I always thought that was a very smart and forward thinking move on the T's part which doesn't happen often. If anything they should try to foster a free/open source app. I doubt the T could be at all competitive TC wise for the front end/native developers, ui/ux, etc needed to make a cross platform app - plus all the support needed. Although, if the T really wanted, they could put a stipulation on commercial apps consuming their data - such as XYZ basic functionality is required to be free irregardless of any paid tier of service, or require the app to pay for the data.