The only thing that should keep up old fleet in place should be experience of these pilots. I don't know if it is implemented in any way, but should be, that those counters of downed UFOs, had some meaning, for example in auto-evade incoming shots/missiles, and (virtually) widening up cones of fire for cannons, or improving critical hits on alien ships (by critical hits, I mean, taking damage score from top of the table, rather than 50-150%). There should be however, possibility, to move pilots BETWEEN planes, as to use the most experienced ones, on already those in full readiness, rather than sitting in base and waiting 2 weeks for theirs planes to be repaired.