Having just watched the film, I would have relaxed into it more if there was some indication why they only launched one ship - ignoring the need for dramatic effect.
Yes, PHM cost $11 trillion or whatever, but once the R&D costs are sunk then building hardware is the cheap bit. Maybe fuel was the issue?
If it's your one big chance to save everything, staking it all on one ship that could get whacked by an asteroid before it even gets out of the system seems foolish.
A single throwaway line about "We've built three but to be honest just one getting there will be a miracle" would have helped, and would have made Grace look like even more of a fluke.
Nice Easter egg right at the end of the credits, though; made me laugh!