You have to trust those miners in aggregate -- they have the power to change the rules, effectively, since the vast majority of network users will follow the majority of hashing power.

Similarly, if any one bank doesn't follow the rules, they get kicked out of the financial network, to similar effect. You don't have to trust a bank, you trust the banking (and legal) system.

If these malicious miners you speak of "change the rules", it creates a fork in the blockchain. Even if they have a majority hash rate, if the economy does not support their decision, the market value of that forked chain will be low while the unchanged fork will keep the market. A group of malicious miners can do very little to destroy bitcoin.

Depends on the sort of change, doesn't it?

Isn't that the soft-fork / hard-fork distinction?

It is true that miners in general cannot use their hashpower to force a hard-fork, but they can, I think, cause a soft-fork.

And, soft-forks can do a lot of stuff.

In what way does the same logic not apply to banks?

Banks can position themselves to benefit from a depreciation of any given currency, miners are tied to the success of the one they've chosen.

Can a miner not distribute their effort and holdings?

Miner hardware is tied to the specific proof-of-work algorithm used in the cryptocurrency they mine. They make extensive capital investments that are, at least in the case of Bitcoin, useless for any other purpose.