[TangoTrotFox]

Decentralization does not mean owned by everybody, it means owned by nobody.

For instance imagine we had a mine that was effectively infinitely large (for the sake of the analogy). If we allowed absolutely anybody with a pickaxe to to come in and mine it, we might call it decentralized. Nobody would own the mine. That one group sent a million miners to go mine it would not suddenly make them owners, even if they happened to receive the largest benefit from the it. By contrast centralization would be when one individual or group forcefully stops, generally with government support, any other individual from mining in "their" mine. In this case they would indeed own the mine. In the case of Bitcoin there is absolutely and literally nothing stopping from somebody from starting up a mass mining farm becoming the new plurality beneficiary.

[Retric]

Due to centralization of hardware the Chinese government can trivially do a 51% attack. That's a form of centralization your ignoring due to an overly specific definition.

The tiny block size is arguably an ongoing 51% attack from an oligarchy that is in collusion. Again, depending on what you consider centralization having a tiny group that's in direct communication easily qualifies.

Pas: In your example if the only option is to use picks from one company or your hands because other tools are confiscated at the enterance then it's very centralized.

[TangoTrotFox]

51% attacks are mostly overblown. If a group decides to engage in a 51% attack, all they are doing is effectively revoking their own license to print money - which is what having a > 51% mining capacity in a major crypto translates to. Bitcoin is a public ledger and double spend transactions would be completely visible to all. All this means is that the coin would end up getting forked, similar to what happened when Ethereum screwed up - and it continued on with minimal fanfare. And in that case their screw up was of their own fault instead of something inherent - even fewer people would be inclined to stay on a Bitcoin line that has been 51%'d.

Rolling with the mine analogy, a 51% attack is equivalent our group that sent in a million miners changing the diamond mine into a 'glass' one. It becomes fake, and loses nearly all of its value and people move to the new 'real' mine. You do nothing except critically hurt yourself.

[Retric]

This assumes the entity wants Bitcoin to survive. The Chinease government has zero intrest in keeping Bitcoin alive and would happily kill it off.

Forks don't help with 51% attacks as the attacker can continue to use their hardware on the fork. Further, you can use a proxy to hid the origin of a block, so Bitcoin would need to move to a new hash which would take a long time and prevent any obvious successor.

[TangoTrotFox]

I was making the assumption that the entity wants to kill the coin. Doing a 51% attack out of greed makes no sense as it'd be immediately self defeating, at least on a mainstay coin.

There are numerous different hashes with varying levels of ASIC 'resistance' - yeah it's a cat and mouse game, but hardly a major issue. The 'obvious' successor would be a matter of the unpredictable public as I'm certain numerous entities would vie to become e.g. Bitcoin 2.0. In the end the market would decide which was the winner. The great thing about it all being that the users could actually come out net winners in the end as the successors would likely not only be technical superior, but it would also be able to use the exact same ledger (as with Bitcoin cash for instance) so that you start with the same relative share in it as you did with the original.

In a way it would even be a good thing as Bitcoin itself is increasingly dated technologically, but is dominant because of its market positioning - which makes it difficult to change. If its market positioning was damaged, we could see the rise of an improved successor.

[Retric]

A few weeks/months/years of not being able to trade Bitcoins at old value is a real downside.

[donjoy]

Besides double spending attack they can also censor certain transactions not allowing them to be confirmed.

[sine]

Ethereum and Bitcoin mining work like this:

Take a freshly baked pie, and give half of it away for a suggested donation of $1 USD to the first 10 people who show up.

Now split the last half of the pie into smaller chunks and slowly give out bread crumbs to new "miners" with the requirement they spend large amounts of capital on exceedingly inefficient number guessing machines in order to win the lottery of "mining" more crumbs.

  Satoshi Nakamoto
  Thu Jan 8 14:27:40 EST 2009
  I made the proof-of-work difficulty ridiculously easy to 
  start with, so for a little while in the beginning a 
  typical PC will be able to generate coins in just a few 
  hours. It'll get a lot harder when competition makes the 
  automatic adjustment drive up the difficulty.

  first 4 years: 10,500,000 coins
  next 4 years: 5,250,000 coins
  next 4 years: 2,625,000 coins
  next 4 years: 1,312,500 coins

The 10,000 BTC pizza is a case study in how Satoshi designed the supply to rapidly dump very cheaply before other users were able to discover the network, by that time and later new users not only need to spend more to mine but less coins are produced for a correct lottery number.

  In economics, the Gini coefficient is the standard measure 
  of how inequitable a society is. This is tricky to 
  determine for Bitcoin, as it's not quiet a "society" in 
  the Gini sense, one person may have multiple addresses and 
  many addresses have been used only once or a few times. 
  (The commonly-cited figure of 0.88 is based on one small 
  exchange in 2011.) However, a Citigroup analysis from 
  early 2014 notes: "47 individuals hold about 30 percent, 
  another 900 a further 20 percent, the next 10,000 about 
  25% and another million about 20%"; and distribution 
  "looks much like the distribution of wealth in North Korea 
  and makes China's and even the US' wealth distribution 
  look like that of a workers' paradise

  Dorit Ron and Adi Shamir found in a 2012 study that only 
  22% of then-existing Bitcoins were in circulation at all, 
  there were a total of 75 active users or businesses with 
  any kind of volume, one (unidentified) user owned a 
  quarter of all Bitcoins in existence, and one large owner 
  was trying to hide their pile by moving it around in 
  thousands of smaller transactions. (Shamir is one of the 
  most renowned cryptographers in the world and the "S" in 
  "RSA encryption")"

Ethereum:

  Presale ICO / Premine ( max cost $0.50 USD per ETH  )
  = 72,009,990 ETH
  
  Total Supply today (Feb 23rd 2018)
   = 97,800,000 ETH

  Source:
  https://etherscan.io/stat/supply