In all honesty I think that this assessment is close but not completely on target!
Personally, and this is only my opinion, I think VZW is making this change to keep their network from getting overwhelmed just like AT&T's network did thanks of a large influx of iPhone users.
Lets assume for a moment that about 100,000 current AT&T users are planning to make the jump when the iPhone 5 hits VZW. In a perfect world this number of new users would be evenly distributed across their network and would not have a significant impact. This, however, isn't a perfect world. It's likely that large numbers of these new users will be located in the nations larger metro areas: NYC, Los Angeles, Chicago, etc. Since iPhone users tend to use lots of data this would likely create a negative impact on the data network in these areas.
By establishing a tiered plan, they accomplish several things. First they keep these new users from gobbling up all of their limited data resources since now each GB will cost these new users money to use. Second they establish a model that would allow them to accumulate more cash flow in those areas where data usage is highest. This could be used to bolster data capabilities in these areas. And third, they could use this as a way to attempt to control the amount of growth in their data network.
The rumor that existing unlimited data users would be grandfathered into an unlimited data plan tends to support my theory. After all if they were truly mercenary then they'd force us all into tiered plans from day one.
In the past, VZW's exec's have said that they are not looking to just have the largest wireless network. Assuming that there is some truth to this corporate rhetoric, this would also tend to support my theory. After all if they wanted the largest network it would be counter-intuitive to implement a tiered data plan which would handcuff their new users and limit their abilities to use their new smartphones to it's fullest potential.