[littlestymaar]

> Mars colonization doesn't make much sense, but building O'Neill Cylinders when humanity runs out of space or energy on Earth might.

Space on earth isn't going to run out anytime soon (as I said colonizing Siberia and Antarctica is much more approachable, and there's the oceans too!) especially since the world population is plateauing!

> but if you've exhausted all the matter in the solar system or humanity outlives useful solar output from the sun and fusion energy is still +20 years away

That “if” really feels like “I'll need a second hat if I grow a second head in the night”.

> The time it takes for interstellar travel doesn't make sense on human time scales, but people still plant trees knowing they will not live to sit under their shade.

That's a good analogy, because individuals do, but for-profits mostly plant trees with short lifetime and quick return on investment. And you're not going to fund space travel with individuals' money…