I think the gun is usable, it just doesn't make economic sense to try to use it. With the collapse of the USSR, the anticipated need for this new class evaporated, and with it the value in manufacturing the ammunition. If the new gun had real targets, they could have justified the expense of the new ammo, and realized per-unit cost savings by manufacturing the ammo in large quantities. However, without such targets, there's no point in manufacturing a few rounds at very high costs, for what is now essentially a prototype and technology testbed.
The Zumwalts made a lot of sense when they were first designed. But the world changed, and they stopped making sense. That's we only have three of them, instead of a dozen, and why we've curtailed a lot of the planned spending to complete their original design goals. And that's why they look silly today, if you don't take into account the context in which they emerged.