Well, the user response if 51% of miners decide to pick a fork that enriches themselves at the expense of users is "alright, I'm done, this cryptocurrency is worthless. Have fun spending electricity to mine these digital bits that don't mean anything."

The users who actually decide which currencies have value are those who are willing to exchange those currencies for things of value - either directly for goods & services, or indirectly through trading it with fiat currencies that you can exchange for goods & services. Like all currencies, Ethereum has no intrinsic value - it's just bits in a bunch of computers across the globe, the same as dollars are these days. Its value comes from peoples' willingness to exchange it for things they do value, which isn't going to be very high if the sellers don't believe it'll be worth something to them in the future.