It was just an off-the-cuff Fermi estimate, though, so I wouldn't bet any probability mass on it.

I'm not sure what you mean by "bet any probability mass on it" -- one of the things about probability mass is that it's conserved... -- but there are many cases in which I'd be quite happy to adjust my probabilities substantially and/or make large bets on the basis of off-the-cuff Fermi estimates. The best reason for not basing one's actions on such an estimate isn't that their results are no use, but that one can very often improve them somewhat with little effort.

In this instance, the main reason I'd be reluctant to base anything important on such an estimate is that it seems like improvements in measuring equipment, and/or just measuring for a long time, might be able to overcome the problem. But if the estimate were (1) a matter of what's fundamentally possible, (2) separated from what present technology can do by several orders of magnitude, and (3) apparently quite robust (i.e., the approximations it uses don't look too bad) then it might be entirely reasonable to conclude from it that Pr(WBE in the foreseeable future) is extremely small.