In short decentralization for bitcoin is like anonymity for mixers you just add another layer of security for investments but this doesn't mean that mixers don't record your data like chipmixer did, and also it doesn't mean that bitcoin is 100% decentralized as you saw it just needs huge power to have it ruined and that is a good reason to use it
Binance was about to ruin it and they wanted to do it but they couldn't do it obviously, and though it's possible to gain control of the blockchain if you acquire 51% of the total mining power, it's not a joke to be able to achieve that especially with how many people and companies are mining these days, the difficulty to gain that much mining power is too high and it would cost too much money for someone to do that, and even after that, one cannot really have access to anyone's wallets but they can only have access to incoming and outgoing transactions.
And Bitcoin is called decentralized because it was created and is running independently, there is no one that can make any changes to the original blockchain or control the coins in circulation or those that are mined, so basically, it is decentralized, and all that you say are just possibilities with no actual outcome seen so far.
