That's odd. I don't always play on the same difficulty each time, but I usually find that the Altarians and Torians ally and eventually destroy the evil civs.
BTW, there is no alignment requirement for Death Knights. Once you research Battlship Technology, you can build as many as you want, regardless of alignment. Just how many Death Knights get built is usually dependant on production ability.
If your game doesn't end fairly quickly, eventually Avatar ships get researched, and the reign of the Death Knights ends, as they become obsolete.
As for how long old ships like corvettes can be found with the AI empires is usually determined by how often that empire is at war. Obsolete ships are pretty likely to die in protracted wars, and modern ships get built to replace them.
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