Is Bitcoin truly decentralized? Explanation below
Short Answer: The control is mostly with few hands.

Long Answer:

Even though in expectation the block generation may seem somewhat reasonable with 0.01%, the variance is quite high. Also, the mathematical puzzle miners are solving continuously follows the property called memoryless. 

In brief, the memoryless property of a puzzle is say you failed to solve the puzzle till time t_0, then the probability that you will at sometime later is equivalent to a fresh start at time t_0, in other words, the failed past effort does not increase any chance of winning a block in future.

So, to reduce variance miners come together to form pools to reduce the variance. Coming to centralization the current hash rate distribution can be found in (https://blockchain.info/pools).

The intermediate between total centralization point comes a classic attack call 51% attack(some close instances in history can be found https://www.coindesk.com/bitcoin-mining-detente-ghash-io-51-issue/). The caveat with 51% attack is that the stakeholders may lose trust in bitcoin in case of 51% attack which will reduce the value of bitcoin which might not be beneficial to miner considering the effort required to launch a 51% attack.