Of course usage patterns will change. I would not try and rape my account on the last day of the month but rather in the last month of my 3 year period.
Not theft at all. You are paying for expected usage. This is how the ISP business model works. The same principle explained in the article will apply. The thing to remember is that you buy gigs whereas the ISPs do not buy gigs from their upstream providers, they buy capacity/bandwidth for that month in mbps based on that expected demand and some magic formula that will ensure that peak times are not severely overcrowded. If no one uses their data for 3 years, the ISP will still need to foot the bill for the capacity they acquired each month. With prepaid data they factor in this possibility and hence prepaid rollover data is more $$$ from eg. Axxess/WA and the likes.
Eg. Say 245 people buys 100GB of 'usage' for a certain month. So they eg. rent a OC 3 (155Mbps) pipe that allows ~47.89 TB of throughput if run at full throttle for the whole month. That is about double than the expected usage during the whole month to allow for more simultaneous connections. Whether or not those 245 people use that data or not, the cost of the OC3 line will remain for the ISP.