Maps is table stakes for a smartphone, and having such a key feature provided by your main competitor is a huge risk. So purely on that basis, it could be worth it.

Then, on top of that, there is value in the data you're able to collect. Traffic data is really valuable. Tracking the movement of vehicles and pedestrians lets you create very accurate maps based on "real world" data, you could use it to figure out really specific things like traffic light timings, diversions, pedestrian crossings, parking space, layout of private roads...

At one point, Apple was working on a car, if you were making a self driving car, all that data would be useful for you, and beacuse of the value of it, competitors may not even sell it to you. So your only option is to generate it yourself.

As for transit data, that is fairly simple, most transit agencies will publish their timetables in GTFS format, there are tools to automatically export this in scheduling software. That will probably get your 90% of the way there, so you might have a few on the groud people in major cities to tweak and make it more accurate, which is nothing for a company on the scale of Apple.