They already use tiered plans. I get a "25 Mbit" connection. That's a tier. Over a month, that works out to a theoretical cap of 8.6 terabytes. That's a cap. Just one that I'll never reach.
Which brings me to my complaint of the day. If I'm paying for 25 Mbit, then give me 25 Mbit. Always. Unless the host can't dish out content fast enough, I want my 25 Mbit. If it's impossible to guarantee me 25 Mbit all the time, then don't call it 25 Mbit. I don't care how fast it can theoretically go if nobody else is using it, I care about how fast it is when everyone is using it, because that's what it will likely be when I'm using it. If it's actually just 2.5 Mbit when under full load, then tell me that.
Also, I'm pretty sure that 52% isn't a number, it's a percentage. It's a large percentage, but a small number. Torrenting requires a lot of upload bandwidth from a small number of people, regular web usage requires a small bit of upload bandwidth from a large number of people. Is it any surprise that the two take about the same amount? Downloading one 4 gig HD movie = 4+ gigs worth of uploads from other people, which works out to a lot of Tweets, Google searches, Facebook minigames, and the occasional audio/video/photo attachment.
So in short, no. It doesn't mean that a lot of users are uploading content. Just that the content is very big.
Finally, I'm calling BS on most people having Gigabit by 2021. You know what my peak download bandwidth was in 2001? About 16 Mbit. You know what it is now? About 24 Mbit. I'll be happy if I break 100 Mbit by 2021.