As with any war, the evolution of the Galactic Civil War means a change of tactics, technology, and policy in order to address new requirements. However, just because a (very small) piece of evidence highlights one particular technology does not make this technology universal, or even particularly widespread.
This is the first mistake that Eamon Hamilton makes in his recent article comparing the newest model of X-wing to the (rather less airworthy) F-35. By relying on mere seconds of film and the availability of a toy, we can only determine that the T-70 is distinct from its predecessor, and that it can fly and shoot lasers. We have no real grasp of its new advantages or capabilities, and most importantly we do not know if it is the only starfighter at the Resistance’s disposal, or even if it is particularly widespread. We could also watch the Top Gun trailer, and conclude that the F-14 was the Navy’s only combat aircraft. The T-70s we see may even be an experimental or elite squadron, which, given the appearance of a new superweapon, more than merits its deployment. The T-70 might be exceptionally expensive, and deployed in a proportion more like the F-22 than F-35. As an aside, though, I think a better comparison would be between the F-16 Block 60 and the T-70, given that both are improvements to 30+ year old designs, rather than entirely new airframes (spaceframes?).
This comparison, however, is fundamentally flawed, and comparing the Rebellion or Resistance’s requirements to those of the US Navy is folly. Doctrine is entirely different: for example, our carriers do not have any significant armament other than their complement of fighters, and certainly don’t fight other capital ships at point blank range. There is no equivalent ship in any fleet on this planet, let alone in the USN. This is partly a consequence of politics. With planets (or moons) being the primary political unit, space is the only medium of travel between them.
This means the Rebel navy is far more important than its American equivalent, where air and land provide alternative avenues. The priority of ship defense reflects this, and the logistical aspect can be seen in starfighter design. Rebel starfighters, for example, are very space efficient: they take off and land vertically, making it easier to arm a ship of a given size. Underlining all this is the fact that the X-wing is vastly more capable and durable than any aircraft currently in service.
As we saw in The Empire Strikes Back, the X-wing is capable of sitting around in a swamp for extended periods of time (days? weeks? a month or more?), sinking into said swamp, and after being raised fly away with little-to-no maintenance. Thus, it is safe to assume its regular maintenance requirements are very few indeed. Barring battle damage, which would be partly obviated by its shields, a pilot could fly the fighter continuously for a week (the duration of its life support systems). Equally importantly, the X-wing can travel literally astronomical distances unassisted by capital ships. Further easing its logistical requirements, its primary ammunition comes directly from its power generator. Hyperdrive and shields are present on all Alliance starfighters, and it is safe to assume that the minute logistical requirements follow with them. This removes perhaps the greatest advantage of moving to a single platform, and also highlights that the difference in mobility between terrestrial vs. carrier-based fighters is not so severe in Star Wars as on Earth. This results in a lot of “tooth” and very little “tail”.
Have some things changed between A New Hope and The Force Awakens? Inevitably, but I find it unlikely that the widening of the civil war (as the Rebellion moves to establish itself) would diminish the need for starfighters with different capabilities. Indeed, it seems that the threats the Resistance faces are not, tactically, much different than when it was still the Rebellion. As the trailer suggests, the First Order’s decisions on how they fight have not changed much from the Empire’s. There is still a preference for TIEs, Star Destroyers, storm troopers, and superweapons with trenches. Given the Empire’s reliance on conventional military structures, there is every reason to believe that most other Imperial remnants remained doctrinally similar. There is also no reason to believe that the Resistance’s position is secure enough that they can safely scrap their less advanced fighter designs. I can think of another conflict that looked rather worse for the side that 30 years earlier were the unqualified victors.
We should also consider the tactical advantages of multiple starfighter types. In a galaxy where dogfighting is the main way of engaging other starfighters, improved electronics and warheads cannot take you very far. This means there is still a clear need for a more dedicated interceptor and space superiority fighter design. The A-wing was designed to fill these roles by the time of the Battle of Endor, and an upgraded version should be available to the Resistance, or even an entirely new fighter.
Hamilton does not help his case by his clear unfamiliarity with the starfighters. He claims the A-wing to be designed for “hit-and-run” attacks on convoys, when, as mentioned above, it was designed to fight other starfighters first and foremost, especially as a counter to the TIE Interceptor. With only two laser cannons and concussion missiles instead of the more powerful proton torpedoes, it is not nearly as capable at destroying heavier targets as the other Rebel starfighters. Hyperdrive makes even the Y-wing capable of hit-and-run tactics, and, with its heavier payload, far more effective against these targets.
Stepping back for a minute, there is a good chance that the T-70 will be the only, or almost the only, starfighter fielded by the Resistance in this film. This is not because of any in-universe logistical or training priority, but rather because having only two types of starfighter (especially those with shapes as distinct and recognizable as the X-wing and TIE Fighter) makes it easier to distinguish the good guys from the bad guys in Abrams’ preferred style, which involves rapid camera movements in close fights. It also makes an obvious callback to A New Hope, and nostalgia is a rather effective moneymaker. Nobody remembers the Y-wings from that film, of course, because all they did was die.
Robert of Bellême (Count of Ponthieu, 3rd Earl of Shrewsbury, etc.), is an Anglo-Norman magnate renowned for his cruelty and violence, who now spends his time posting bad jokes. His favorite starfighter is the TIE Interceptor.