Miners can only double spend transactions if they have > 50 % of the hash rate.
The only purpose of miners is to add proof of work to in exchange for new coins that users value. If the proof of work is on a chain that contains valid transactions and conforms to consensus rules, it'll be valuable.
https://bitcoin.stackexchange.com/questions/55316/what-happe...
> Clients will ignore invalid blocks when determining which chain is difficultywise-longest. Mining clients will not build on top of invalid blocks. Clients will not propagate invalid blocks (or invalid transactions).
Proof of work does not mean full nodes trust miners blindly. It only means they prefer the longest chain that follows the rules.
Like I said, yes, with 50% mining power you can double-spend all you want. But other nodes know what you are doing, expel you from the network, and discard your work, just as if you had 25% of hash power or 0.1%. The Bitcoin blockchain is not a democratic system where the majority always wins.
If someone with 1% hash power ignores certain blocks, they make a chain that's totally irrelevant in their own corner. If someone with 60% hash power does it, they win.
Bitcoin users can always choose a different PoW algorithm if a threat actor coerces too high of a percentage of SHA256 mining equipment