Bitcoin is becoming CENTRALIZED and is INSECURE
More than 50% of the mining is done by Chinese pools. That means that they control the network - who's transactions get in, who's transactions are left out. 


Sure. But electing to censor transactions is pretty much equal to you refusing to pick up some unwanted dollar bill on the ground. Last I checked, eventually someone would pick up free money. 


  Since the block reward was reduced to 12.5BTC, they are completing it with transaction fees - the bigger the transaction you're sending - the bigger the fee you need to pay them. Last time I've checked I've seen some transactions with fees of more than $1000 worth of BTC in them. So in a sense, Bitcoin is starting to act like a traditional banking system - owned by Chinese people.


False. As a user, you are in complete control of how much fees you want to pay. You can pay the minimum fee rate so as long as you have the patience to wait for the transaction to be confirmed. Alternatively, if enough users elect to pay a fee rate that is homogeneous, miners are still incentivised to mine transactions that pay lower fee rates.


  How can the network continue to be P2P like that? And especially what happends if that main storage will be taken over by the authorities and be shutdown?


If storage is an issue, you can elect to run your bitcoin node in prune mode, which effectively removes the need to maintain the whole blockchain. AFAIK, there are enough full nodes with the blockchain out there to prevent a state attacker from shutting down all of them.


  What happends if tommorow blockchain.com gets shutdown for legal actions? What will happend with people's wallets and their funds then?


Users should never trust web-wallets nor hold coins in web services to begin with. It was an unintended usage scenario.

Instead, users with storage and computational constraints may elect to store their bitcoins in an SPV wallet, offline paper wallet or hardware wallet with varying privacy/usability tradeoffs.


  If I was the owner of these pools, what can prevent me from deliberately ceasing all my mining power.. wait for the net difficulty to lower, and then do this attack?


The net difficulty will not lower itself until a full epoch (2016 blocks) has been reached. So there is no incentive to cease mining operations for the above reason. In fact, there is a huge opportunity cost to be paid by a major pool that ceases mining operation, consider the lost revenue opportunity. That alone could potentially drive a major pool bankrupt.