Q11: Will we be able to upgrade our relic weapons to accommodate the increase of the maximum item level?

A11: You will be able to upgrade your relic through the “Saga of the Zodiac Weapons.” While the whole process will be very long, the appearance of your weapon will change if you upgrade your relic weapons up to a certain point.
Hi,
Sure they don't expect everybody to do it but there are people that want long grinding.
The upgrade is not mandatory and there are other weapons iLvL 100 to 115 that don't demand that amount of grinding.
It's a choice, I guess they allow people to have easily all Zenith relics but to get all zodiacs not that easy.