I don't think of it as a stretch at all. It is twice the size, twice the mass, and twice the price of the Connie, yet carries less cargo and has smaller turrets. Seems obvious to ask what you get instead of what comes with a Connie.
I dunno what makes you think 5-10 years but that's not even close. They won't start this for another year, but it will be done in two--probably by Beta time. I would not wait for it, but it's nothing like 5-10 years. I'm not happy the Prowler won't be ready until Beta, but that would not stop me buying one nor playing the game with a Hoplite. Anyone who wants a 600 can buy a Connie or a Carrack in the meantime. By the time it is available, you can buy the 600 with in game cash.