No one would have predicted that a B-Wing would be in a Rebel Aces release regarding game reasons but there it is. It's a very popular ship. I'm sure it won out over the Y-Wing for that very reason.
Actually, the B-wing makes a lot of sense for an "aces" expansion. It's a very popular ship right now, but only as a generic ship. The named characters aren't seeing much use, and even the generic ships are locked into a choice of "advanced sensors and/or HLC". An aces expansion gives FFG an opportunity to fix this and make named characters more viable with a powerful and exciting ability on a character everyone's been waiting to see, and a cheaper low-PS character that lets you put a character B-wing in your list without spending 30+ points. And of course it does all of this with the "awesome" factor of a high-end ship with access to fancy tricks that other ships can only dream of.
The Y-wing, on the other hand, isn't as obviously a good choice as people think. It has its flaws, but those flaws are inherent problems with trying to make the Y-wing be more than a point-efficient ion platform. It would be a lot harder to "fix" the Y-wing without more of a redesign than you can do with a couple new characters and a title card. And the Y-wing aces would probably still lack the "wow" factor even they're good at winning games. It's a poor dogfighter, so you're stuck with the boring-but-practical roles of ion turrets and torpedo delivery.
(The obvious choice for an "aces" expansion was of course A-wing and X-wing, but FFG already did the X-wing side of it with the transport.)
It leads me to believe that epic scale ISN'T locked into 1/270, as was stated. Fudging will occur and you can stick with the orginal dog fighting game if you feel that strongly about it. The larger kits won't effect your gaming experience at all.
It's not locked in completely, but it is locked in within a reasonable margin. FFG fudged the scaling a bit with the big ships we know about, but they did it in a fairly subtle way. The ships are smaller and cheaper to buy, but on the table they still look about right compared to the other ships. You won't notice the problem unless you're a hardcore fan with the patience to analyze them looking for scale issues. A star destroyer (or similar huge ship), on the other hand, would have obvious scale issues that even casual fans would notice as soon as you put it on the table. Star destroyers are awesome because they're huge looming wedges of death, a star destroyer that's hardly bigger than a ship that's supposed to be almost invisibly small against its huge bulk is going to be a hopeless disappointment.
And that's on top of the rule scaling issues. The corvette and transport work because they're roughly in scale power-wise with a handful of fighters. A star destroyer would either have to be crippled to the point that a practical number of fighters can be a relevant opponent for it, in which case it's a disappointing ship that nobody really likes, or accompanied by new rules where capital ships are mandatory, fighters represent whole squadrons, etc.
It's been said that the larger ships aren't locked in at 1/270 and scaled more towards "playability".
A small number of fighters can indeed be a danger to a single Star Destroyer. We'll see how they handle Epic play. Each fighter representing a squadron, lead by that character etc makes sense. It would be a more "tactical" approach to the game and in those regards, scale can be sacrificed to a degree.