The good news is that even in the event mining takes a big hit for some reason, due to the decentralized nature of Bitcoin there'll be next to no issue with having at least one person being able to mine and (hopefully) be able to confirm all the transactions that would be going through.
I doubt there would ever be an issue with this though.
If I understand it right, the network will be vulnerable to attacks if there are less honest nodes in the network. If large miners suddenly stopped mining, then attackers will have a chance to mess up the chain or insert invalid transactions if they have enough power to overcome the remaining miners.
As such, the verification is reliable as long as honest nodes control the network, but is more vulnerable if the network is overpowered by an attacker.
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https://bitcoin.org/bitcoin.pdfHonestly, I do not think large mining farms will quit suddenly since they are heavily invested. But if they do, I think it will be bad for bitcoin.