We actually just got 55 Mbps down / 5 Mbps up from DOCSIS 3.0. I can't imagine needing anything faster in the future, but I'm sure in 10 years I'll think it's slow as molasses.

Luckily DOCSIS3 scales pretty well. You can just keep adding bonded channels. 4 or 8 channel D3 modems are common these days but we have 16 and 32 channel models waiting in the wings. Since cable systems aren't exactly symmetrical from a spectrum standpoint the upstream speeds will continue to lag a bit but 25-50Mbit/sec is do-able today. 50-100Mbit will probably be common in 5 years.

I'm curious about why the artificial limit? The cable cos have enough backbone bandwidth, right? So why can't they just add channels and sell 100mbps+ today?

Some companies do offer 100Mbit packages today however it's mostly a gimmick. They would face a much bigger challenge/cost to offer every single customer 100Mbit. There's a surprising lack of demand for customers who are willing to pay more for faster speeds. It seems the $50 or less per month price point is what people want.