I wouldn't be surprised if the new X-wing model in VII is smaller (and more compact) than the ones used the the Civil War.
Hence the E-wing...
The E-wing is expensive. It's a F-14 to relate this to 70/80s USA fighters. When faced with the risk of high losses, a general is going to send in the less expensive, multi-purpose fighters (F-18 and F-16). The more expensive fighters, like F-15s and F-14s, will be fewer in number and supported by the general fighters.
We see a similar shift from A New Hope to ROTJ. The Y-wing is out in force to support the X-wings in the battle of Yavin. Then, at Endor, the X-wing has taken the Y-wing's roll and specialty ships like A-wings and B-wings are the strategic fighters that are being supported by the X-wings.
30 years later, much of the X-wings technology will be common and very cheap. They will be like the Z-wings in current game. The X-wing of VII will be better than the ones seen in ROTJ, but still old technology compared to the new hotness... upgraded E-wings or whatever it might be.
Fun to think about... but can't wait to actually see the movie!