If I take a GUN, and aim it to Vitalik's head (ETH), or Justin's Head (TRX), or Changpeng (Binance) head, or Anatoly (SOL) head, telling them to shut down their network, that the game is over for them.
Wrong, they are not game over. Did you read how blockchain's working today? if there was no critical problem with it and it can still alive but the problem is if those blockchains didn't run by a lot of nodes. That means when these nodes will be done and blockchain will be stopped.
Can they do it?
They can't do that. The network has been running by so many nodes. It will never stop the blockchain. It can be stopped when there was no node active in the network and that will be shutdown the blockchain.