Investors != Miners. If they are investors, they may not necessary be the ones mining at the pool either. The thing about these pools is that they are NOT exclusively tailored for a small number of miners, but they are tailored to accommodate for a large number of miners with their own farms. That alone should tell you how decentralized Bitcoin mining is.

The FUD that is put forth in this thread is largely unwarranted. The key factor that you're ignoring is with the game theory that the miners are involved in when they invest their resources into mining. The key question would lie with whether they gain more when they act maliciously, or honestly. It would be obvious that the answer is to be honest. Selfish mining is largely only a concern among pools, because they are able to gain an unfair advantage than the rest of the pools. It poses much less of a security threat than you think.