Bitcoin is somewhat decentralized depending in which perspective you view it from. As a system that allows anyone to use it for sending and receiving money around the world without any external interference, it is considered a decentralized cryptocurrency. However, in terms of mining the cryptocurrency, it is somewhat centralized if you tell me. Due to its current mining algorithm, high end hardware has been designed that only those which have enough capital to afford them, could participate in the mining process.
As Bitmain is the number one manufacturer of ASIC hardware, it gives them power to control most of Bitcoin's total mining hashrate. So, in the case that Bitmain would become shutdown by the Chinese government, or the company decides to perform an attack on Bitcoin, then the pioneer cryptocurrency would be doomed.
That is one of the biggest failures Bitcoin has in terms of decentralization which would need to address before it’s too late. But, despite everything, Bitcoin is considered as a decentralized cryptocurrency when compared against traditional monetary systems. Just my thoughts