The thing with the Colossus is that people want it to do what it's not meant to do. It doesn't dart in nose-to-nose with Shivan destroyers because that's not how it was designed. Just look at the turret layout for proof. You've got six huge beam cannons down each the port and starboard sides. It's like ye warships of olde (you know, the ones that float on the water) that maneuver to fire from the broadside of itself into the broadside of another ship. Put three Ravanas on each side of the Colossus, and they're meat. Put them in front of or behind the Colossus, and you've got a six kilometer long hunk of Terran-Vasudan scrap. You might want to note that that statement holds true for a lot of Terran ship designs. Just look at the turret placement on the Deimos, Iceni, Orion, etc. Most, if not all of the anti-capitalship punch comes from cannons mounted on the sides. Any ship is a lot more effective when the mission designer knows how it should be used.
As for starting construction of a new Colossus immediately after Capella's destruction, it's a bad idea. The GTVA just lost an entire star system, along with a decent chunk of its workforce and consumer population. Maybe economic depression is an overstatement of what is likely to come, but the GTVA is definately doomed to a hard recession. Perhaps you could come up with the resources and labor, but if so, it would be better to use those resources to strengthen the economy before building a warship that isn't immediately necessary.
That leads to the next point. The Shivans are gone for an indefinate period, and the NTF has been crushed. For the moment, you're in a state of galactic peace, so there is no necessity for a huge warship to be built. This is a situation where a wise politician would be putting money into anything possible to get civilian manufacturing rebuilt and the number of taxpayers back up (the GTVA did just lose a whole lot of them, after all).
Finally, even if production time is halved due to equipment and designs already being prepared, the Colossus was really a twenty year old design before it even saw combat. Now, in present day navies, if a ship has run for twenty years, it's served its time and is due to be replaced by something less....well....obsolete. Building a new Colossus after the Capella incident would be roughly equivilant to me buying a Pentium 75MHz priced as new.
All said, a new Colossus would be a really bad idea. I've seen some better ideas rolling about campaigns in development though. New ship classes are probably the best idea. Carriers are a nice idea, as Juggernauts would have a hard time standing up against a determined squadron of Helios-equipped bombers with fighter escort. (Remember, future heavy bombers would likely be designed with the Helios in mind, simply because its bigger and badder than the Cyclops. As for bombers being slow, juggernauts don't exactly outpace the Perseus either.
) Another idea would be increased specialization in capital ships. If you strip most of the anti-fighter defenses off a corvette, you can allocate more power to anti-warship beams, giving you a nifty little frigate capable of downing far larger capital ships. Likewise, if you yank the anti-warship equipment from a cruiser, you can beef up power to engines and tack on even more anti-fighter turrets to make an escort frigate.
The Colossus was a demonstration that bigger is not always better (though, it is more impressive when you see it just jump in and vape a corvette in one shot). Now, before you tout the Sathanas, remember it's not much better in any way. It's just designed for the kind of frontal assault that some people like to put juggernauts up to. I daresay the Sathanas is actually more vulnerable to bomber attack, but that's a train of thought for another day.
