You can easily make relics upgradable, perhaps give them a new look each upgrade to signify you still have the best weapon on the server. If you noticed, most of the relic weapons right now do not reflect ultimate weapons in FF lore, they have been around in the power house, but not actually ultimate. It could be made to upgrade relics to a whole new ultimate weapon.

Curtana is more a recent name, but you still have it upgraded to something like Excalibur, Lightbringer, Caladbolg, and so on.

Holy Lance, Kain's Lance, or Gungnir for DRG.

For how many people have relics right now (including myself), I would assume they will make more powerful weapons than the relics. I do also believe however they just won't let them collect dust, I am willing to bet that they will be upgradable per expansion.