What may seem easy to an outstanding person is most definitely not easy in reality.
There is so much more complexity involved in this, and not even speaking of the whole corporate nightmare new features have to go through.
okay. if they had never done it maybe. but its in Google maps already. So they already have all the data available. Making the code to show transit, with having access to their internal API's is probably very easy.
Just look up a local bus provider API. Very easy to just make a request. Google probably has their own API already that bundles all transits from over the world.
Crap, I forgot we were on a thread about smart watches. I wouldn't know how it works on one, since I don't own any.
But turk by turn works as good as it can for any public transport service. It tells me when and where I need to change. How long the change is and things like that. I assume a lot of it depends on the data provided by the transport company
Your last sentence is the only valid complication, all the other issues exist just the same on phones and Google has already solved them in their existing apps.