You are making the assumption that conservatives will just stay home. Not a single conservative I have talked to said that is what they are going to do. In fact, most of us who will not vote for Romney will vote GOP down ticket and write in a protest vote for president.
I'm making my assumptions based only on electoral history. The best measure of voter intensity in a Presidential election year is the strength of the Presidential candidates themselves. If Obama cruises to reelection, as many FReepers assume he will, history shows that there is a great likelihood of Democrat candidates being elected as well.
My point is not to be fooled into thinking that denying Romney office to send a message to the GOP establishment is free of down-ballot consequences; punishing the establishment may turn out to be punishing overselves.