With its value , people thinks bitcoin is mainly a financial thing . But REALISTICALLY, it is a network security that is govern by thousand of decentralized miners. It is a pretty hard and expensive to hack a system like that.
I´m not sure if you have understood the white paper correctly. On the final page, where the odds of cracking up the blocks are given, the numbers are not for one block, but for the hacker catching up the whole blockchain.
As you can not know, what happens inside the mining pools (they just share computing power), then REALISTICALLY there is no guarantee, that your coins stay with you. And it is far from being decentralized, by now, I would say, it is extremely centralized already.