Bitcoin has decentralized aspects, but I don't know how long this will still last.  Of course, there was a strong centralization in the protocol, by Core.  But for a long time Core respected essentially the immutability apart from technical issues that don't matter much: bitcoin was *economically* working like it was lined out in Satoshi's paper, and Core hardly changed anything to that.

You have to distinguish the *software* written by Core and the protocol adhered to by that software.  The protocol has only undergone small changes that were of very technical nature, and didn't actually impact anybody at the moment of the modifications ; in other words, those aspects of the protocol were essentially unrealized and hence open to modification.

So, in as much as there was "leadership", that leadership didn't modify the protocol in essential ways, and was hence equivalent to its absence.

In functioning, bitcoin went from highly centralized (Satoshi and a few guys), to very decentralized, to back rather centralized with 5 or 10 or 14 mining pools.  On the other hand, Core, trying to impose its leadership, actually lost it and now on the protocol side, bitcoin is in fact more decentralized (over 5, 10 or 14 pools) than when Core had de facto leadership.

Is a group of 5, 10 or 14 antagonists decentralized or not ?  In as much as they don't collude, in fact, yes.

You can have a decentralized system from the moment that you have 3 antagonists that can't come to an agreement.  But it is fragile, and you never know if there is collusion or not.

So, bitcoin is still somewhat decentralized in as much as these 5 - 10 - 14 entities do not collude, which nobody really knows.  On the protocol side, bitcoin got just more decentralized with the fight between classic, Core and BU, because it wasn't and there was leadership, but there isn't much any more, which is good.