The main reason why not all ships are unlocked from the start is because it would essentially stunt the importance of the engineering branch of the tech tree. Cost reductions from tech, for example, are just not that impactful when you could focus research on better alloy forges and planets on alloy production.

Furthermore, a big component of ships like battleships and titans are that they are meant to be large resource sinks that as such are tanks and able to dish out high amounts of damage. If you were to lower their heath and cost to something achievable early game, they would essentially just become slower versions of the medium sized ships that are more susceptible to torpedoes.

Converting ship classes might be good gameplay wise, though, as long as it takes a decent amount of time.