"it would be just as easy for a Democrat with more appeal to the left to unseat her in the old-style partisan primary than in California's non-partisan primary."You'll have to actually make a case here, rather than simply asserting it. In a general election, F vs. DwiF, with Republicans breaking to DwiF, F has much less of a downhill battle than typical.
"Strategically voting Republicans would be taking a pretty big risk of doing that, because that means they stand a good chance of ending up with a general electing of DwiF vs. F, and the realistic anti-surveillance Democratic candidates tend to be more opposed to Republican positions in every other area than Feinstein is."
They stand a good chance of yet another Republican losing to Feinstein, if they all vote for the Republican. Depending on who DwiF is, that may or may not be preferred. It may very well be that there's no one with the right positioning in the field at present; there very well could be by 2014.
None of this is to say that Feinstein is guaranteed (or even likely) to lose in 2014; simply that pointing out that the last races weren't close doesn't tell us much, and I think there is more of a chance then there has been.
Of course, she's also 80, so it could plausibly even be that she retires before we find out.