We're talking about surface vessels, here. What layer can a drone even occupy? Close-in intercepts are handled with CIWS systems, BMD is covered by VLS, AA handled by SAMs. Drones do not handle any of those mission profiles better than their counterparts, cost be damned.

I'm entirely willing to write off drone interceptors for the foreseeable future. Layered defense certainly presents opportunities, but not for low-reliability expendable doodads.