my theory has always been they didn't want us to get the technology, so they only put it on the one invincable super ship
I've also had some other interesting Shivan theories, one was similar to the Shivan technological grabbag theory.
that the anchent destroyers did not actualy destroy the anchents, but that the anchents won, and 8,000 years later now they are the destroyers, untill they get into a fight with us, and after we take them on we will eventualy become the next Shivans.
I don't follow that theory, and never realy did, but I always thought it was a fun one.
current theory of the Shivans, they are simply infinite as far as we are able to see, controleing vast swaths of the universe (note I didn't say galixy, and oh, yes I know what I'm saying) lets say something like 2% of all mater in the universe is Shivan technolody, dyson sphere like power suplys built around induced black holes. we are insects to them, the ships we've been fighting are construction equipment, designed not to destroy, but to fine tune the galixie's naturaly caotic gravity/subspace feild and enableing them to make stableized contact with the real Shivan armada. think about it, what if the Sathanus is the shivan eqivilent of a garden rake? they arived in the milkyway sevral hundred thousand years ago and have been reshapeing it into something a little more conducive to inter galactic subspace travle.