There's nothing stopping Bitmain from centralizing Bitcoin anytime soon. Slowly but surely, Bitmain has been having a negative influence over the pioneer cryptocurrency's hashrate. This becomes worse when no other ASIC hardware manufacturer could outpace Bitmain in terms of energy efficiency and performance.
By effectively creating a monopoly over Bitcoin's mining process with pools being centralized, there's a huge probability that Bitcoin would become a centralized cryptocurrency in the future. Developers and the community would need to work together to stop this from becoming a reality or else it will be too late for the world's biggest cryptocurrency in market cap.
Nevertheless, I believe that is in the people's interest to keep Bitcoin as decentralized as possible. However, only time will tell us what lies ahead with the longevity of the most prominent cryptocurrency in existence to date. Just my thoughts