I believe the idea of the Black fleet crisis was to show the new generation of the new republic. B-Wings, A-Wings Y-Wings and X-Wings were now obsolete, as were the old mon cal cruisers and other material. The idea was that as time progressed, so too would technology. I mean by the time the Black Fleet crisis occurs the X-Wing is 20 years old, the E-Wing was built to replace it and thus the Recon X, where the X-Wing wasn't good enough to serve front line anymore, and so they relegated it to a support role (For a real life example, as the F-15 replaced the older F-4, the F-4 was still used as a reconnaissance place)
However fans liked their old fighters too much, and so the E-Wing and K-Wing were forgotten about, and more than 40 years after the X-Wing was built the new republic is still using them in a front line capacity. It does stretch believability a bit, even though they are upgraded. Why don't we see refitted F-104s and MiG-17s used in the front lines of the best militaries in the world these days?
Just some food for thought.