Many apps will prompt before doing a large download over data. Spotify has separate settings for mobile data and Wi-Fi streaming quality. One could imagine a video app would prompt before streaming on mobile data. I'm pretty sure this is the solution—perhaps the Android or iPhone media framework itself could implement something that would warn people if app developers are often forgetting to add this feature?
I think they're close to a good solution but not quite there. A tri-state button, where it's on/on-but-disconnected/off might have done it, or at least some indicator that "off" doesn't mean off.
No, it does not if the Wifi Chipset is disabled, because it uses wifi for location services (GPS would use way too much battery). That's exactly the problem: The wifi chipset is used for much more than just connecting to the internet.
You can also use the GSM Cell ID for this purpose. Once the user marks his/her home, grab the IDs of the surrounding cell towers and use these as trigger.
It did, but I'm not sure how well it really works. In my own experience, I still see lots of networking failures if I'm far enough from my house for the network to be dodgy but not so far that it disconnects, or if I connect to crappy public WiFi.
I'm not aware of any option in this country (edit: the US) that will handle massive use of streaming video. All of the plans without overage charges have a soft limit where they start throttling you.
In any case, I'm not going to pay a bunch of extra money every month just in case I forget about the WiFi.
1. You're in a coffee shop. The WiFi sucks today. You turn off WiFi so you can use your cellular connection instead.
2. Many hours later, you go home, having forgotten about #1.
3. You binge-watch the entirety of Doctor Who streaming on your phone, not realizing the phone is still using cellular.
4. Large bill from your provider.