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.