No, Bitcoin is still decentralized. It is an internal property of Bitcoin. The only thing big groups can do is manipulate the price by buying/selling.
This is probably what @mrquackquack meant when he spoke about the decentralization of bitcoin (although this is not entirely correct), when large holders of bitcoin can influence the price and manipulate the market.
Yes, I think that we continue to think about the same classic definition of decentralization in this topic, but the OP meant something else. He has already been warned about the terminological confusion, so it would be more constructive if we focused on answering his question.
Yes, I'm worried about more and more Bitcoins being held by such groups as it makes it easier for them to manipulate the market. If we were worried years ago about the power of miner farms, now we have additional risks to worry about.