It does have existing missile cells. They have their role. But the Zumwalt was “sold” as a land attack specialist with special long-range guns that could shell a target area on land in preparation for a Marine landing, or similar event.
If it effectively has no guns, then it has lost it’s primary mission. It’s not “useless” (because it has 80 missile cells) but it is somewhat pointless.
It’s now an underarmed, hideously expensive version of the 90s Arsenal Ship