Some cumulative fatigue. Some age hardening. But, for the most part, an older rim that isn't already failing (no cracked spoke holes) should last a while. Maybe not forever, but certainly good for going out on few hundred mile rides.

Perhaps the biggest difference is stronger profiles and better alloys in the newer rims.

If you're doing your own work, I'd just go ahead and buy $30 worth of spokes and put it back together. Always buy name-brand spokes.

If you're having the wheel built, then certainly consider parts value vs labor costs.