Or they just use Phalanx-like [0] close-range defensive aid suites which already exist?
Sure, that will consume a lot of bullets but swarms of defensive kamikaze drones will also get used up and need resupply. In which case the resupply ships themselves also become high-value targets.
I thought the Phalanx was exactly the type of defense system that drone swarms were more or less designed to penetrate. Sure, it can put out an unreasonable amount of lead (tungsten? uranium?) downrange but the number of disparate targets it can engage per second is actually quite limited. It's vulnerable to being swarmed.
I agree that a Phalanx can be swarmed but I'm having problems envisaging drone-on-drone attritional combat involving a capital ship lasting for vey long. If you (capital ship owner) are attacking a peer state it can probably afford to swarm you repeatedly over several days also throwing in plenty of additional standoff missiles, perhaps something ballistic, a sub or two, probably others I haven't thought of.
You'd probably get away with it handling intermittent Houthi attacks in the Red Sea but in a serious war I think the logistics would get you in the end.
There are multiple different drones. I bet phalanx will do nothing against small 10" drones which can target communications and sensors of the ship in a first wave. And second wave could have much heavier payload to target ship's structure
Sure, that will consume a lot of bullets but swarms of defensive kamikaze drones will also get used up and need resupply. In which case the resupply ships themselves also become high-value targets.
[0] https://en.wikipedia.org/wiki/Phalanx_CIWS