But does everything have to happen in our lifetimes to be good/useful?

I often think and plan for the future both for my family and beyond,
as I am sure you do. You wouldn't stop saving money for your children
and their children's future just because you might not be around to
see them use it, right? Indeed you may have a stronger desire to
plan for such a future because of your natural limited time with them
on this planet.

Besides, one day we may be able to live far longer than the measly
few decades we do now. I think it's a good idea all around, as I feel
like I am just getting my act together in my forties and the fact that
I will probably have major system failures just a few decades from now
I find deeply unfair, as should many of you, even the ones who got
their acts together in their thirties. I no longer consider anyone in
their twenties - especially their early twenties - to have their acts
together, and if they do, it's probably more like some kind of premature
physiological atrophy.

Anyway - The more advanced you are, or if your biology already allows
you to live rather long, the stronger your desire and the better your
ability to survive for many centuries or more. I presume this for humanity
as well as any ETI of a sufficiently high level.

One thing I don't agree with you - lacking any exact numbers based on
what you said - is that we won't be around by the time a message we send
to an ETI gets a response back to us. If they are in the Virgo Cluster, maybe.
But if they are anywhere near us (if there are 1 million technological civilizations
in the Milky Way galaxy as Sagan once estimated, then the nearest one on
average is 200 light years from Earth), then we as a civilization could have
a long but actual conversation with each other.

If we plan for the future as I stated at the beginning, then not only will the
human race survive for itself and any ETI who might want to talk with us,
but we will eventually evolve via AI and related technology to become (or
create) beings that will survive and advance indefinitely. This is what I also
suspect the ETI we eventually encounter will be like as well. For them, as
with things on a galactic scale, conversations that last for millennia will be
neither extraordinary nor daunting.

We need to get the ball rolling, as it were, if not for us, then for our future
selves and the future inhabitants of our galaxy. I for one do not consider that
a to be a dead-end effort.