That is nonesense really, sorry to say. They made a choice to add some of the old ships, and leave others out. If they later do add one, it's still going to be a new ship, even though it might look the same. Because of the work that's gone it. It's been made again, had to be programmed again, tested again, etc.
Have you got any idea how much work and money goes into making the ships suitable for the new game and finally delivering them as an addon? The ships from the old game aren't compatible and can't just be put in. Surely you have noticed that they're quite different now? They all needed a major overhaul. That means a lot of work, and work costs money. Simple as that.
The developers all need to eat too. Example: If you have the old iphone, and the new version comes out a few years later, do they give that away for free to all the people that had the old version of it? I think not. Same thing, only an addon for a game is much less expensive.
Regards,
Fred