you are right, seems BTC is loosing decentralization
but that just means... nothing can be decentralised for real
Only primitive organisms. Worms for example.
Ants have primitive centralization. They can build an anthill.
People can work in fully centralized community. They are launching rockets to Mars.
BTC lost its decentralization when some smart guy decided to mine on his video card and another clever guy organized a pool.
The development of bitcoin was never decentralised, looking at it from a wider angle, the blockchain itself is a centralised system(the blockchain) on which we entrust the bitcoin economy. The meaning of decentralisation is that there is not a single entity but the blockchain itself, until there is someone who can make and overtake a mining rig so powerfull it will mine all the remaining blocks and will not share the technology.
This will however result in a non trustworthy blockchain and the whole thing crumbles before the one investing so hard to gain control will have the biggest loss. "Now you're king of the mountain, but it's all garbage!"