Here's my counterpoint to each of those three theories:
Maybe Google is trying to force Adwords buying on certain phrases.
The "Adwords theory" has been voiced since the beginning days of Adwords, and Google has always passionately denied doing that. In fact, playing around with their core competence (organic search) in that way would destroy Google. Over the years some companies have analyzed huge piles of data hoping to catch Google in a manipulation like this, and it never has held any water.
Maybe Google is trying to give several equally deserving sites their time at the top of the US SERPs
I'd say your counterpoint is right on the money. Intentional ranking rotation would show ups and downs in total traffic, not non-converting zombie traffic at the same essential level. In addition, the webmaster could pretty easily catch it in action - and so far no one has.
Maybe Google is testing different taxonomy profiles
And as we both agree, such taxonomies testing would not show patterns that last for years.
I don't have a site under my management that shows a throttling or zombie pattern of any kind, so I can't really dig into any data myself. I'm hoping someone who feels they have this issue will look more closely at it, segmenting by keyword, geography and time of day - if this is not just some mirage it seems to me a pattern should start to show itself.