There is a whole culture built around Google's search engine policies with some practices being labeled "black hat" for evil marketing practices that allow people to pay their way into a top search engine ranking and "white hat" practices that are deemed more natural estimates of a site's reputation. In the grand scheme of things, there are many gray areas, and "gray hat" practices that can be allowed even though they may be commercially motivated and that's hard for any search engine to spot.
However, Google does try to keep people from buying their way into top rankings as this would make their search engine ineffective and help them to lose market share. Then, someone else might be cleverer in delivering search engine pages that match a higher level of integrity in search matching that isn't influenced by commercial interests. That's why when Google decides to penalize people for tricks (whether black or white hat) that help them to place better in SERPs, there are many cries of dismay when the rules change. They mean serious business and won't be upset if you lose your livelihood in order for them to keep their business alive.
However, no matter what rules Google puts in place, Yahoo! and MSN are their own separate search engines. Their rules may be different. And, the myriad of other smaller search engines too may not be following the same standards that Google tries to enforce on the Internet. So, despite the fact that Google is a major player right now, it doesn't mean they will forever dominate the Internet search engine business.
Try to follow Google's rules as best as you can without alienating potentially other profitable ways to do business on the web such as gray or white hat tricks of the trade. Google might penalize your for a paid backlink or review, while Yahoo! and MSN couldn't care less. They are just one player in a very large, and dynamic, environment that is constantly changing.
Who knows if Google will be there tomorrow or not? Since it is the BIGGEST it probably will. But hopefully, your focus is on keeping yourself alive on the Internet and making sure your business is healthy while not tripping any live wires out there that other Internet entities rig for their benefit.