Anybody with 51% could hard-fork Bitcoin, holding everybody's blocks hostage.
Do you even understand the concept of a hard fork? It's unrelated to hashrate. A hard fork occurs when the consensus rules are altered, allowing a previously invalid transaction to become valid. For example, if you validate a block subsidy of 100 BTC, you've initiated your own hard fork. Good luck persuading others to follow you.
And no, it's not possible for someone malicious with 51% of the hashrate to validate invalid transactions. Miners are only capable of arranging the sequence of transactions. If a malicious miner controls over 51%, they could opt to mine empty blocks or reverse transactions, effectively undermining the fundamental principles and game theory behind the system. If Bitcoin were subjected to such an attack, it would cease to function. There would be no remaining mechanism for control. Get it now?
Do you even understand how software works? Code can be changed.
And China would absolutely "persuade others to follow them" because they would be "persuading" a bunch of servers running in computer room. There are no people involved in the Bitcoin system, which is the whole point. You have this wrong-headed notion that Bitcoin is some kind of "community", which, while on a daily basis it is in places like here, that "community" could be overridden by an entity with more servers. Bitcoin's "consensus" is not among human beings, it's among anonymous servers. Plug in a majority of those to the network, and whoever or whatever controls those systems controls Bitcoin.
If there was a "central governing body" of Bitcoin, like you keep implying there is, then Bitcoin would not be decentralized in the way it's advertised to be. That committee could simply be compelled to do whatever governments wanted them to do.
And yes, if China (or whomever or whatever) took over Bitcoin, people would grumble a lot, but most would be so terrified of losing their holdings that they'd do anything the entity wanted. The price would surely go down, but every major holder of Bitcoin would be running advertisements 24/7 saying, "everything is okay". (So in a way, Bitcoin is a PoS model after all
).Its technically possible to control that 50% and requires huge investments to achieve, and btw in the case of China you make this sound like it's one state owned entity running the show when in reality it's several independent entities that represent this virtual control under the umbrella of being labeled China
Yes, but at some level China comes down to one controlling entity, and even one man. Think of their nuclear weapons, for instance. There's not some rogue faction somewhere in China who could launch some of those.
And whomever controls over 50% of the hashrate can dictate the software in use, and/or dictate the contents of the blockchain. As the theory goes, if China wanted to wreak havoc on the world economy, or if it wanted to extract a ransom, or if it simply wanted to profit by inserting its own blocks, then... it could. They could do anything.
It's actually more complicated that you make it sound, think of it like a car.. 
