Do you know how many text emails you can send in 200 MB? Likely something over 10,000. Over ten thousand text-only emails.
There's no freakin' way you can convince me that you (or any other natural human being) would ever send and receive anywhere close to a total of ten thousand text-only emails on their smartphone within the space of one month.
So clearly, even after we subtract out all the text-only emails you (or any other natural human being) would ever use within the space of one month, there would still be space left over for other uses. And that's before we even start considering the people who elect for the 2GB plan.
Your analysis also ignores the fact that both plans still include unlimited WiFi usage in any AT&T hot spot. This will encourage people to take advantage of WiFi where it is available, and that is actually a good thing in terms of overall efficiency of service -- it provides faster speeds and lower latency to customers at the hot spots, and it frees up cellular bandwidth for the customers who are not near a hot spot.
Here's a nice little FACT for you:
Even with these caps in place, the VAST majority of customers (98%) will be able to continue using the iPhone the EXACT SAME way as they always had under an unlimited service, except now they'll be paying less for the same amount of usage. Let me repeat that so that it sinks in: They will not need to put any thought or effort into attempting to reduce their usage at all -- they will continue using their iPhones the exact same as always, and pay less.