To come up with the figure of 2 decades, Shostak took the Drake Equation and crunched some numbers. He figures that n is somewhere between 10,000 and one million in our galaxy.
I don't know what's crazier, Shostak's take on the Drake Equation or his utter disregard for time scales. Given recent insight into astrobiology and cosmology, most experts put n at 10 or less. Some, like myself, put n at less than 1. But 10,000 to 1,000,000? Holy geez. I can't believe a guy from SETI is actually suggesting this. Sounds like he needs to dig into Ward and Brownlee's book, Rare Earth: Why Complex Life Is Uncommon in the Universe. Sometimes I wonder just how tuned into reality SETI people really are.
Moreover, if Shostak seriously believes this, then he has some rather strong opinions about i) when intelligence could first arise in our galaxy, ii) ET's inability to engage in interstellar travel, and iii) a civ's inability to distribute Von Neumann probes. What Shostak may be subscribing to--and I haven't heard him be this explicit about it--is the phase transition model of the universe which suggests that the conditions for the rise of intelligent radio transmitting life have only been established recently. In other words, intelligence in the galaxy has only been able to get going recently, which is why we don't find the entire galaxy colonized by post Singularity intelligences already. But like I said, he hasn't actually said this, so I may be giving him too much credit.
If Shostak believes in the phase transition model, then he joins a number of other cosmologists and astrosociobiologists, including Serbian cosmologist Milan Cirkovic, who believe that this is the answer to the mysterious Fermi Paradox. These thinkers speculate that the next phase of the universe will necessarily involve the rise and union of multiple intelligent civs--a prospect that wasn't physically possible prior to this point in the universe's history.
For those that reject the phase transition model, ie those who believe that the galaxy has been able to produce Von Neumann producing civs for millennia, they argue that the detection of ETIs would be a very bad sign. Philosopher Nick Bostrom is one of them, who in his paper, Existential Risks, claims that:
The probability of running into aliens any time soon appears to be very small... If things go well, however, and we develop into an intergalactic civilization, we may well one day in the distant future encounter aliens. If they were hostile and if (for some unknown reason) they had significantly better technology than we will have then, they may begin the process of conquering us. Alternatively, if they trigger a phase transition of the vacuum through their high-energy physics experiments (see the Bangs section) we may one day face the consequences. Because the spatial extent of our civilization at that stage would likely be very large, the conquest or destruction would take relatively long to complete, making this scenario a whimper rather than a bang. ...Thus, as Cirkovic notes, we are led into a bizarre situation that out of all scientific disciplines, astrobiology is the only one whose successes are not desirable. This particularly applies to the SETI sector of the astrobiological endeavor. This being said, Cirkovic rejects Bostrom's theory in favour of the phase transition model, as outlined in his paper, On the Importance of SETI for Transhumanism.
There must be (at least) one Great Filter – an evolutionary step that is extremely improbable – somewhere on the line between Earth-like planet and colonizing-in-detectable-ways civilization . If the Great Filter isn’t in our past, we must fear it in our (near) future. Maybe nearly every civilization that develops a certain level of technology causes its own extinction.
Luckily, what we know about our evolutionary past is consistent with the hypothesis that the Great Filter is behind us. ... This would change dramatically if we discovered traces of life (whether extinct or not) on other planets. Such a discovery would be bad news. Finding a relatively advanced life-form (multicellular organisms) would be especially depressing.
Of course, Shostak may be unconcerned (or blissfully unaware) about any of this. His proclamation may merely be an attempt to secure future funding for SETI, which would be sad from the perspective of sound science.