Rank 14 is the final rank for veteran rewards.

[Alberel]

Maybe they're working on something new to replace the Veteran Rewards system that they just haven't said anything about yet?

Yoshi actually already confirmed that they are working on a replacement system. If I were to guess they'll likely have some kind of special NPC merchant that sells veteran rewards for a currency that you earn each month instead of having fixed rewards. This way newer players can get the rewards they specifically want whilst long-term vets will simply have everything.