Rooting around the corpora, I have found a couple more examples of throwing/pouring cold water on hopes, so it seems that some speakers at least are happy with it.
. In some regards, your corpus citations fail because they focus on hope. It's as if "I washed my dog/horse/car is OK, but I washed my alligator/hippopotamus/barbie Doll is not OK because it doesn't come up that often. That seems odd to me.
That's a poor analogy. You can
wash just about anything, just as you can, in the literal sense of the words,
throw/pour cold water on just about anything. The number of things you can
throw cold water on , in the sense of
reducing enthusiasm for /
criticising is severely limited. I have searched over two million words, and found that the most common collocation with this expression is
idea(s). There are almost as many citations for
idea(s) as there are for the total number of citations for
proposal(s)
, hypothesis, effort(s), speculation, plan(s), dream(s) and
hope(s).