Bzzz! Hold it right there! What is "sufficient bandwidth"
My ISP has advertised 100mbit service, and I have every right to expect that 100mbit will be available 100% of the time. ISPs oversubscribe their actual available bandwidth because they know almost no one ever uses 100% of the available bandwidth 100% of the time. That doesn't change the fact that they are charging multiple customers for the same resource. The ISPs can't then turn around and say, that there isn't enough because their customers are using more than their fair share, when in fact, the ISP has sold more than they had available in the first place.
Using that metric, Sufficient bandwidth is whatever is required to provide 100% of their customers with 100% of the promised bandwidth. Anything less than that is just the ISP whining because they are being held to the contract they themselves wrote.
In that regard, ISPs with data caps should be required to advertise the datacap / billing period instead of the peak speeds, customers will quickly stop coming in the door when it is made obvious that a 50GB / month limit effectively means that on average you can only get 150kb/s download speed over the course of an entire month. If the ISP had to advertise that 150kb/s instead of being able to claim 100mb/s speeds, they would quickly change their minds about data caps.