Well, for one, the doomed endeavors (if it actually is) are often worthwhile for the following reasons:

-The technology that stems from attempting to answer the problem.

-The unforeseen consequences - i.e. Maybe someday we'll reach technology that can detect signals in a more advanced way than we can currently and, through recording data from the past, have captured a signal years before that technology exists.

-How many people buy into lotteries despite having awful odds? The odds may be bad, but it's non-zero.

> The technology we have to send signals today will look completely pathetic to us when we look back on them in 50 years.

the technology doesn't matter, either yesterday, today or tomorrow it will still be electromagnetic waves which is what SETI is analyzing.

There is no reason to think we will be using grossly non-directional broadcast EM waves in 50 years for communication, and certainly not in 100 years. It's far more likely we will have extremely high bandwidth over very low distances, since that allows for far more bandwidth in aggregate (since you aren't interfering with other users on the same spectrum).

Or we might have moved on to laser (highly directional) or something nobody has thought of yet. I promise you we won't be using some kind of nondirectional, multi trillion watt carrier-based radio contraption that we would have to be using to be spotted by something like SETI on alien world.