Conspiracy theories don't die becuase it's obvious to reasonable people that manipulation is happening, so it's not a conspiracy theory. Search for a popular e-commerce item and see.
The idea that the average user will be upset enough to leave to Bing if Macy's shows up #1 for "wedding rings" vs another store that is not a Google advertiser is a joke. Most will click on the ads /products that make 90% of the page anyway. Others can easily scroll. Rankings would change normally anyway, why not make some money while at it?
Why hasn't anyone come clean? Mmmm...maybe because it's not done in "manipulate organic search to earn another $0.875 billion this quarter" memos but indirectly like "what if we favor brands to sort the web? Wow! Great results. You're a Google Fellow now. Brilliant!". Those penalized will have to spend a fortune to get back up and the current brands even more to maintain their rankings, like buying ads even if you are #1 almost as insurance.
Even then that is a poor defense, no Googler came out that they were breaking the law on pharma ads, or that they were openly stealing code from Oracle and taking chances of being sued instad of paying for it, or that Android is being used to force phone makers to carry Google products, etc. Only legal discovery got them out, not the supposedly superior ethics of Google employees. God knows what they are hiding. The google myth is being exposed and no one buys the crap Google said in 1999 or 2007, people are looking at results and at what they do.
They have been exposed more than once as liars, arrogant thieves and plain greedy b@stards, all while playing the underdog card and bombarding us with propaganda. That's the truth.