The large majority of users are kids. They don't go on subways or planes often.
Look at Pokemon Go. It requires an internet connection at all times. And yet, it's been insanely successful. Imagine that.
but they DO go on road trips and car rides with mom and dad or to the park or hangout with buddies down the block (yes, kids do still go outside even though this generation is always looking at a screen). there is no wi-fi there unless you are in places like a major city such as bryant park new york city. parents will not allow unlimited data usage as we all know those unlimited data plans are a lie. after a point, you no longer have unlimited data as speed is throttled and all you can do is check email. i don't see how this is viable. when i was a kid, i took my portable gaming systems with me. i didn't stay home just so i could play a game. that's what a real console was for.
and a good % of kids do go on airplane rides as well as in situations where there is no internet. i already mentioned that an internet connection also kills battery while playing the game compared to throwing the phone on airplane mode and playing for a longer time.