Honestly SETI is a waste. Its looking for radio waves transmitted by a civilization at exactly a specific point of radio development (which lasted under 100 years for humanity) while also advertising ourselves to potential hostiles...
I don't understand why you think that radio communication will suddenly go away. If we assume that our current understanding of physics is correct, radio is always going to be around because:
1) Being light, it travels as fast as anything in the universe
2) It isn't easily absorbed by stuff that gets in its way (the interstellar medium, the atmosphere of a planet)
3) It's the lowest energy form of light, so it's relatively easy to produce
Therefore, it's somewhat likely that we will be able to detect radio from a technologically advanced civilization. That does assume that there isn't some sort of magic (ie sufficiently advanced technology) based on physics that we don't currently know, but I think that any reasonable discussion has to assume that our understanding of physics is at least somewhat correct.
Now, humans are moving away from easily detectable radio signals, which is somewhat problematic. But I don't think that its reasonable to assume that all aliens will move in the same direction with their technology. Some of them might not move toward directional communication for any number of reasons.
So yeah, there are assumptions going on that lesson the chances of detecting aliens, but the price tag on the project is hardly large by any reasonable standard.