The problem with arsenal ships is twofold. One, we don’t have enough missiles to fill the ships we have now, let alone a ship specialized to carry as many missiles as possible. Two, missiles and ships are like eggs and baskets. To counteract the egg/basket problem you want to distribute your missiles among multiple ships, and then equip those ships with sensors and defensive systems so they’re harder to sink and so you can develop a detailed picture of the battlespace. We have a name for that kind of ship; it’s a guided missile destroyer.
You can go smaller (that’s called a guided missile frigate) but thanks to the square cube law, a bigger ship can carry much more than twice as much missiles, sensors, etc. as two ships that are half the size. You also get economies of scale on maintenance. There are tradeoffs here, and the DDG as the backbone of the surface fleet and the guardian of the aircraft carrier is a great solution to those tradeoffs.
Don’t be a doomer when it comes to the arms race between missiles and air defense. Air defense can defeat a concentrated missile/drone attack, as we’ve seen in the two recent Iranian attacks on Israel. There are novel air attack threats from drones, but there are also novel air defense systems to counter them such as lasers (https://x.com/osinttechnical/status/1886259012632535520). If I had to guess I’d say that the balance is actually shifting in favor of air defense on net.
I do agree with you that submarines are important. In fact the Ohio class SSGN practically is an arsenal ship. The problem with submarines is that they have to operate as lone wolves. Surface ships and carrier aircraft can share sensor data and use sensor fusion to construct a shared picture of the battlespace, but submarines can’t be looped into the network without exposing themselves. Again, tradeoffs. And usually the solution to these tradeoffs is combined arms—you build a task force with a carrier, some surface ships, and some submarines and each ship has its own job to do to cover the weaknesses of the others.