It should definitely exist, but Apple's one-size-fits-all approach is pretty dumb. It ought to be configurable. The 100MB limit is way too big for someone on a really limited cellular plan, and way too small for someone on a high-end plan.

It should be as simple as a toggle to turn it off.

You're typing from a first world country where there are not huge cellular costs or speed limitations. If you go to other countries, even say, Botswana, the option to download over a gig in apps every (other) week isn't cheap. Bandwidth costs.

Don't worry, you can get to third world carrier really quickly by taking a Greyhound bus to Canada.

As a Canadian, to get 2.5 GB of data from a larger cell phone company costs me over $100 / month (there are smaller companies that will reduce that, but still, it's little data for quite a bit of money). I'm very happy I can force things to download only over WiFi.

>(something I still can't believe exists in 2017

Something that I'm very happy about in 2017 :-) Data is expensive in many places, and wifi/broadband is typically cheaper.