For Quora and StackOverflow, they can (and do) index personal blogs etc., too. Google is a search engine; they don't need a single fixed third party to be the first search result to all questions in a category.
And if you want it, Google sees most credit card transactions. In 2017, they said, "Google’s third-party partnerships ... capture approximately 70% of credit and debit card transactions in the United States." https://adwords.googleblog.com/2017/05/powering-ads-and-anal... (Later reporting found a deal with MasterCard, at least.) So they could absolutely know if you bought the Amazon item or booked the hotel, if they wanted.
I wholeheartedly agree that there's a centralization concern about letting one company have so much power, but that doesn't change that there doesn't seem to be much advantage in another party being the review aggregator. If we can't figure out how to return control to the actual reviewers, Google is no worse of an aggregator.
There are distinct advantages to having a single cloud, with a single set of protocols/APIs/etc. The only issue I see is if Google starts filtering/blocking in a significant way, then the blocked communities will build their own infrastructure. But Google wouldn't want that data, since they are filtering it out in the first place, except perhaps to train their filters more accurately.
Again, ianal and dont really know what im talking about