Shouldn't the question be, is bitcoin still centralised? Because, Bitcoiner as I am, and as un-technical as I can be, it's impossible to deny Bitcoin started out centralised -- but that's just the nature of something that has to come out from somewhere and be born. The software was run first by one, then two people, and then a handful of pre-early adopters. Might even have been dominated by a few developers and enthusiasts a year after, still centralised. They were the users, the miners, the developers.
It's nothing like that today, hasn't been for many years. Even the first user, the first centraliser, the founder -- he walked away so this dream of decentralisation could happen.
You sound like complete different from what is my understanding on decentralization. Even, when only one person was using bitcoin it was still a decentralized one because the mining reward belongs to anyone who has more hash power (the second person may join at any time and the first person has no authority to stop the 2nd one from joining). I mean to say we need to consider what are the things which define decentralization within bitcoins: the production of bitcoin is not controlled by an individual nor by a group and no one got power to revert/charge-back a transaction or to freeze an address and its balance. I strongly believe these are the features which define that bitcoin is a decentralized and definitely not by how many users/miners/developers are involving.
Regardless of 1 or 1000s of people involving, the feature I have mentioned will not get changed. So, it is always decentralized so far. Without identifying what belongs to decentralized, it will not make sense debating whether it is still decentralized or not.
You missed my second response which was about the political centralisation, rather than the architecture (which, mind you, if operated by 1 or 2 people still means the decisions would be made by them).
You talk about only 1 person using bitcoin but you have to remember that 1 person at the time had also to run the actual client, full node and verify the transactions and include it into the new blocks. Imagine a theoretical today, where only 1 person was doing that on a full node. if 1 million new users theoretically today only join only as users, and that one guy decides to shut down his computer, you're not going to get magic blocks every 10 minutes.
Anyway, there's even enough evidence without going to the extremes of Satoshi or Hal Finney sitting alone firing up the network to demonstrate that Bitcoin was centralised at first.
A mining company called Ghash in 2014 found themselves unwittingly in ownership of "
powers that circumvented the decentralisation" when their mining pool got so popular they crossed the 51% hashrate threshold several times. I wasn't around at the time, but they apparently voluntary shut down to prevent this.
Again, I'm totally saying Bitcoin is decentralised today. But this privilege did not come without challenges, nor should it ever be taken for granted.