Google's dominance creates a monoculture, so that any weakness in their ranking is immediately exploited by SEO folks. If there were multiple real search engines using different ranking algos, it would be harder to game them all at once, which would shift the balance somewhat towards just making good content.
Even with multiple search engines, if they are all trying to solve the same problem, you'll get convergent evolution.
Let's say you have 10 search engines each with their own algorithm. 1 of them has an exploitable weakness. Either they close this weakness by switching to one of the superior algorithms or they get exploited and become much less useable than their competitors. In either case, you soon have 9 search algorithms. Repeat this process a few times and you're back to a monoculture. Best case scenario, you reach a stable equilibrium where a few different search engines are all plagued by different but roughly equivalent issues, such that none is good enough to beat out the others.
