The thing is that once you get that corvette you will find yourself taking the scenic route on your way back home, and doing more things with this new found speed that you otherwise wouldn't have done. It's human nature.... technically having faster speed will not increase your data usage... but throw in the human element and once you have this speedier data, you will tend to do things that you didn't do before thus adding to your data usage.
Yes, of course it opens up new doors. keeping with the car example, you can now go race on the weekends/take a back road like you said/etc. But you don't HAVE to do those things. You can. You have the ability. And if you do, you pay.
From a different perspective though, I can't think of one thing that I currently CAN NOT DO at all on a 3G connection that you CAN ONLY DO on a 4G connection. Are there certain apps that ONLY work on 4G? (of course not yet). I just don't see how, suddenly with a 4G connection, I am going to start streaming movies to my phone every day (or any other task that uses a lot of data).
I think most people will continue to use their devices in a similar way to how they use them now... Sure there will be the occasional "I did this because I can on 4G" thing, but I don't think that will be commonplace. The same with the vehicle example. Sure there will be days when you're driving your new sports car that you'll say "I'm gonna open it up and go fast today" but day-to-day, you're still going to drive like a normal car.
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while this is true, i don't think habits will change that significantly. i already have the ability to do things like watch HD youtube videos on my phone... but i rarely do it... and won't start doing it because i have a few extra mbps to work with.