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.
Sure they can buy off the devs but once those devs go against the Bitcoin community, they will no longer be accepted as devs.
If this becomes known to the
BTC-community, and before that moment there is time to do harm.
In fact, anyone can be a dev. All you have to do is make a contribution to Bitcoin code which the larger part of the community agrees with.
How many active
BTC-developers are there at the moment? Probably a few dozen at most?
There is another point regarding decentralization: the majority of the hashrate (computing power) is in the hands of only a few mining companies, which also doesn't benefit the discussions about decentralization.