What's the point? It's not going to occur in a reasonable amount of time, and if you say, "Well, eventually it will..." then eventually everything that exists will cease to exist anyway.
Your argument can be further extended as "what's the point of living when eventually everything will cease to exist anyway".
But ultimately, it's these small wins along the way that help push humanity forward. I'm not going to open the "what's the purpose of life" can of worms but to dismiss scientific research because of how long it will take is a defeatist attitude. You can not predict what fruits will a particular scientific research will be bring just as you cannot predict the future.
The point is that we don't know exactly when an unpreventable extinction-level event will happen. It makes sense to plan for such an event as early as possible.
May I remind you the dinosaurs died because they had neither nukes nor rockets.
Nor telescopes, but that's beyond the point.
Mankind puts all its eggs in one basket. Our odds of survival against planetary catastrophes increase with the number of planets we colonize. The more spread we are, the bigger the chances. The timescales involved are huge but, just like someone wins the lottery every week, I'm willing to bet that, as we discuss, a civilization we know nothing of is being wiped out by an unforeseen catastrophe precisely because all of them lived on top of a single rock.
Of what comfort would a daydream ark be for the rest of us? There are 6.8 billion of us. What good are my feelings of camaraderie if an impactor happens to snuff most of us out? "Oh boy, I've been blasted to ashes by an inbound comet, but it's all OK because someone else made it out alive."
I also dispute the utility calculus you're performing.
At what cost could we move a sustainable colony elsewhere?
What is the probability that we'll all be expunged by a bit of rock zipping around the solar system?
Is there another way we could allocate those resources to achieve a better expected outcome?
> I've been blasted to ashes by an inbound comet, but it's all OK because someone else made it out alive."
Don't be so selfish. Odds are a couple of the survivors are distant cousins, so, at least some of your genes are safe. And you can also record messages for the survivors to safeguard.
Or we can develop the technology to upload you to a simulation somewhere else.
> What is the probability that we'll all be expunged by a bit of rock zipping around the solar system?
Given time, if we stay limited to the Earth, 100%
> Is there another way we could allocate those resources to achieve a better expected outcome?
Again, it depends on the timeframe we are talking about. I think the first thing we have to do is to thoroughly understand the threats we face. The second step is to understand what Plan-B looks like. We really need to look into environmental risks (Yellowstone, climate change), political ones (Iran with nukes), economic ones and the risk of a floating rock with our name on it hitting us soon enough. If a rock or an erupting Yellowstone or a bunch of fanatics with high-tech WMDs don't kill us, we may get fried when the Milky Way and Andromeda collide (it'll happen in about a billion years, and while stars won't likely hit each other too frequently, gas clouds will light up). If that doesn't destroy the Earth, the Sun will eventually swallow it as it gets old, in, IIRC, 6 billion years.
The Earth is doomed. Every planet and star is doomed. Even the universe itself is doomed. Its only chance is if someone smart enough survives long enough to figure out how to avoid it.
Why not focus on things that are problems now?