I was pretty disappointed about the loss of multiple ships at first, too; however I've gradually been warming to the idea as I've been thinking about how same-y all the ships you could fly in X3 were. Sure, there were lots of them, but even the capital ships flew like big, bumbling fighters if I remember correctly. If they've traded that breadth of ships for a deeper, more interesting ship experience, well, I can't say I'm sold, yet, but my fancies are certainly tickled. Especially with the possibility that later installments in the Rebirth series could add more breadth while keeping the depth, though that's quite optimistic of me.
Even better if it flushes out the capital ships as something different that actually feels more capital ship like (at least to me). And with drones taking the place of fighters from what I gather, and prices for ships/drones (probably) not based around providing player progression from small ships to large ships, I'm really interested to see how it turns out. Especially if they give us something really good to do with our fleets in the late game. There were plenty of brick walls to throw your assets at in X3, but the game didn't really feel like it was originally designed with sort of gameplay in mind, and it always felt shoehorned in, to me.
I just hope that I'm not wrongfully excited about this game... and that my old computer can run it.