Suddently becoming centralized (which is impossible) is one of my biggest concerns.
Since it's impossible you have nothing to be concerned about.
Someone acknowledging how to hack / get access to every single wallet thus security would be unexisting.
Impossible if you understand how the network works. You can't decode every single private key without enough time and computing power. It would take years to get even one.
Losing bitcoin value all sudden due to some big shark owners selling as an overall team or group.
He would first have to buy.
There are whales but they know that dumping it all and crashing the price is unprofitable because of low liquidity on exchanges.
They would be able to do it but it would cost them money.
Bitcoin becoming the second biggest coin rather than current 1st.
But who is the competitor? ETH? BCH? They aren't as good and as popular. Also less stable.
Crypto network stops working suddently and every transaction freezes.
Impossible due to decentralized nodes in every part of the world. The only way to make it stop is to cut the Internet but without Internet banks won't work too.
Miners stop producing satoshis -> no transactions -> price fluctuation -> everything dies.
But how are you going to make all miners stop mining when some of them are in the US others in China, some in Iceland and Russia. Bomb the world?
All your concerns are irrelevant.