I think Kieth has mentioned in the past somewhere that it is becoming hard to make things durable in the average case. Even if you set HP and armor of a ship to int32.MaxValue (or whatever type is used for HP and armor), and give it command grade armor, a good sized late-game fleet complete with golems, spirecraft, and spire captial ships (for humans) or Mk. V ships, Mk. V starships, hunter killers, and maybe even a mother ship or two (for the AI) will be able to chew through that pretty quickly.
Average damage * average fleet size has grown sort of out of control, and now its near impossible to make any sort of super-ultra durable ship that can stand up well on its own, if for no other reason then sheer data type restrictions.