This scenario sounds implausible for me on a number of fronts. The resources and personnel extended to do something that could be done remotely, with probes and have no contact at all with us is much more likely. Information gathering does not require the the kind of evolution of a phenomenon that has existed well prior to the golden age of UFO's and is just as robust today in terms of number of sightings and changing forms. That we don't have the same large organizations investigating, nor do we have the same media hype in either reporting or cultural production, and the lack of social/military hype in general, tells us it is simply not a focus for our times. If anything it's a fait accomplis in the minds of the masses. Sure we believe in UFO's; so what?
Still we have to consider that it has been an ongoing, ever shifting phenomenon that is well interwoven with our culture. Reflections on it as a static or focussed singular event don't make sense to me either, but these bursts of the phenomenon as being culturally significant tell me that to understand it requires astute sociological analysis, along with many other disciplines. Case reports continue, just as our affection and consolidation of the past history of UFO's continue to be agents that shape not just what we know about it, in terms of its rhythms and motivations, but even how we experience it. Perhaps my distaste for the ETH is that it asks for these very concrete, or even traditional narratives and speculations, about the history of something we know so little about and have scantly proven, only indirectly at best.
In order for the ETH to work we have to keep inventing possibilities and narratives to fit the theory. I don't have a problem with this approach, after all, with excellent & interesting research Nick Redfern has created a robust career of examining creative possibilities for paranormal events. I would rather ask why is it that something wants us to believe in the reality of extra-terrestrial visitation? Sure it may be more abstract but it also feels a lot more accurate, even if we are to only consider that a fraction of the ongoing sightings are truly unidentified aerial phenomenon, that may be under non-human sentient control, often displaying what appears to be an unearthly technology.