Once you start streaming 4K HD Video, you'll hit that 1 GB cap limit QUICKLY. AT&T is building in the lock-in and gouge plan early.
Well this is the future. It isn't practical to not have data caps if they can't afford the amount of escalating data everyone is using. Data is a cost to the ISP so someone has to pay for it.
If you downloaded at max speed, since there is comms overhead there are approximately 10 bits to a byte. So 1gbit/s => 1024 mbits/s => 102MB/s => 6,144MB/min => 368,640MB/hr => 8,847,360MB/day => 265,420,800 MB/mon => 253TB/mon.
They are saying that if a high end user hits 1TB, it's starting to get uneconomical for them. If they didn't cap it, you would be able to use theoretically 253x more. Pretty sure that is going to cause problems for profit margins! Regardless of if it is included in the plan, internally there has to be an upper limit to what they can afford.
Netflix 4k requires ~ 16Mbps. For a month that is then 16Mbps => 1.6MB/s => 96MB/min => 5,760MB/hr => 138,240MB/day => 3,317,760MB/mon => 3.16TB/mon. So you would be able to stream 4k video for approximately 7.5 hours per day to hit 1TB.