I agree that Bostrom is putting a lot of weight into a very narrow perspective. Something like this seems not only plausible, but extremely likely, IMO. Immortality is probably not going to be based in biology, and just a few hundred years of immortality probably changes the nature of the 'beings' in ways that we cannot begin to comprehend. I simply do not see us looking or living like we do 500 years from now, and 600 years of history is not even a moment in the cosmic timescale. SETI, although noble, is looking for something that would have had to been created for our benefit, which targets an extremely brief and arbitrary moment of our technological evolution. IMO Bostrom's hypothesis suffers from a human-centric, present-centric, bias.3) Physicality may very well be a temporary condition of civilization and once a civilization disappears into the singularity why would they bother attempting to communicate with unenlightened bags of blood?
Jeron Lanier had more than a few things to say about the Singularity. One point he had that I happen to agree with is that liberal atheists cling to it the way conservative Christians cling to the Rapture. That said, the society we live in today is virtually unrecognizable to the society we lived in back in the 50s. Yeah, there are similarities, but we've come a long way, baby.