That's what you might think with a certain naive look at WWII. All indications are that in any modern conflict between countries with the capability to manufacture this kind of hardware, it'll be all over before there's time to design or build anything. You win with what you have when the war breaks out, or you lose. Assuming you're lucky enough for it to not go nuclear, since any country like that can also build lots of nuclear weapons and delivery systems, and they pretty much all have already.
Battleships sided aircraft carriers for two reasons: one was to thicken their inner defense ring a little, and the other was because battleships alone, without a friendly air wing to keep the other side's strike aircraft away, were doomed.
20% better at 3x the cost is usually a bad bargain in a battlefield increasingly dominated by dispersion strategies and that was what eventually killed the Battleships, they were to expensive for convoy duty and too slow and vulnerable to torpedo and air attacks for the line of battle fighting that actually happened doing WWII.
Only because neither the US nor the Japanese had carriers in those engagements. The writing was on the wall when the Brits crippled the Italian gunship fleet with a single obsolete aircraft carrier in 1940.
It's hard to say for sure, but in a way it seems that the Hood and the Prince of Wales actually stood a much better chance of causing serious damage to the Bismarck than the Ark Royal did, even if that's not how events happened to play out. (And in fact damage from the Prince of Wales's guns did help to slow the Bismarck down and force it to attempt to go to port to refuel.)
WW2 battleships proved their worth as late as the 1990s when nothing else could deliver the shore bombardment capability.
While the Phalanx CIWS is inarguably impressive in specification and testing, the thing about the US military is that, whenever we get into a fight, we're always 150% ready - for the last war. It takes us a while to figure out what we're doing in the current one, and it costs us. I see no reason to imagine this pattern shouldn't hold in a new war against an opponent equipped with Moskits.
Think of the problem:
- At Mach 3, the missile is flying at roughly the same speed as the unguided bullets the thing is firing. If it's additionally doing evasive maneuvers it's going to be pretty hard to hit.. (though I guess at Mach 3 it's not going to do very much acrobatics, but perhaps even a little bit is enough to make the CIWS miss?)
- Mach 3 is close to a km per second, and I guess the realistic range for a 20 mm cannon isn't that much, lets say 4 km for the sake of argument. And below, say, 1 km it probably doesn't matter if you keep firing at it or not, it's likely to hit you out of sheer inertia even if you hit it. So that gives you around 3s time to fire and adjust the aim. Not very much..
I have seen some mentions of a move toward missile-based defense, which would probably be a more effective option, but most of the same considerations apply - each missile might be much more likely than an individual CIWS round to score a kill, but many fewer missiles can be carried, and the launchers still need time to reload once they exhaust their magazines.
This is also, incidentally, the real reason why DEWs and hypervelocity kinetic weapons - lasers and railguns - are such a matter of interest in the naval research establishment at the moment. They're justified as being a more cost-effective means of dealing with low-level threats, but this is America - the last time our military services had reasonably to worry about being short of funds was no more recent than the mid-1930s, and nobody since has given any meaningful fraction of a damn about the cost per shot of naval weapons - and frugality would militate against the very considerable R&D cost of new systems without new capabilities, in any case.
A much more plausible reason why these new systems receive such interest is because they might eventually offer a solution to the currently insoluble problem of how to defend against a mass attack with supersonic AShMs. You don't see that discussed much in public, because no one with a stake wants to suggest that our enormously expensive carriers no longer offer the power and prestige that merits the cost of keeping them active, but nothing else makes as much sense.
What you need in a big war against a worthy adversary is the ability to quickly innovate and iterate on designs, and having a single design that try to do everything is about the worst option available for that.
In a war against an adversary that isn't worthy, you can use whatever and you will still win.