A key flaw in the essay is that the failure modes inherent in remaining a single-planet (or even a single-system) species go unstated, and are arguably even worse:
They would know that their fate was
created for them by ancestors who made
the choice to enter the starship, a
choice they could never unmake.
That might be irritating.
In any case, the process of attacking the problems Robinson mentions will make us unimaginably stronger. Perfecting fusion power, understanding and mastering our own biomes, and learning to coexist with ourselves will hardly be a waste of time, even if we never overcome all of the showstoppers he cites.
In fact, I'd agree with Robinson in one key area: humans will never make it to the stars.
Somebody will, though. They won't be humans anymore, not as we are now. But their ancestors will have been. That's us.