The current state of RP, or lack thereof

[Dab]

Correction: X-wing was not used by the empire, because the Rebels stole the 5 prototype X-wings from Incom. So the Empire made a new design, one cheaper, for its fighters, giving Tie Interceptors to veteran pilots.

It has nothing to do with cost. If the Rebels can afford to create them, the
Empire could. The Empire DID make the Tie Defender, the most expensive fighter in the galaxy during its time. Over twice the cost of an X-wing.

B-wing was not even designed until the rebels destroyed both death stars. By that time, Incom was on the Rebel's side, and producing their X-wings.

The ships stolen by the Rebels weren't out of date, or inefficient. The Nebulan-B frigate was a VERY capable escort vessal, both for transports and for escorting larger ships in combat. It can outfight an Imperial Acclamator-Class Star Destroyer on its own. It can give an Imperial Victory-Class Star Destroyer a good fight even.

The Assault Frigate can fight a Victory to a stand still, and kill an Acclamator without breaking a sweat.

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