There's also the serious risks of losing the craft on the way and even discovering that Alpha Centauri is already occupied.

The latter point is in tension with the rest of your argument. "No one colonizes the vast resources of space: they're too crowded" doesn't work as a Fermi Paradox explanation. Uncertainty about one's prospects for successfully colonizing first could modestly diminish expected resource gain, but the more this argument seems persuasive, the more it indicates that potential rivals won't beat you to the punch.