The Collossus' death is symbolic.
It represents humanities (and by extension, the GTVA as awhole) attempts to reclaim the stars - the hopes and dreams of a generation which has lost its home.
When the Collosus is destroyed, it's not just the deaths of 30,000. It's the death of any hope that the Shivans can be defeated. It hammers it home - Capella is doomed.
If the Collosus had survived, what would it be? Nothing more than a failed weapon which could not hold back the Shivans. This way, it went out in a blaze of glory. Had it not been such a fugly ship, it'd have been pretty damn poignant.