I don't know, they might hold the cure to cancer... We destroy our ecosystem at our own peril.

In this time of drought, this seems like a no-brainer, but I would rather spend water on fish in an actual river (where tens of millions of other creatures live) than for cattle or almonds.

Thinking about these actions in aggregate, it can be about losing X gazillion gallons of water to save a species, instead of 4B for 6 fish. I think the point is that these actions add up.

Think of it as the cost of these six and all future fish of this species.

In addition to the difficulty of measuring the cost of losing an entire species forever, as others have pointed out, the question is really do we want this to be an actual functioning river? The more sensitive species are indicators that something has been fundamental changed in the ecosystem.