WT
Ships do wear out. Ticonderoga class cruisers first came online in the early 80s. Arleigh Burke destroyers in the early 90s. Spruance class destroyers are being slowly decommed, as they are even older than the Ticos. Oliver Hazard Perry class frigates have been around 20+ years.
Current manufacturing capability has to be kept running, or we loose it. Only one place builds subs for us now, and we are in a world of hurt if it were to close down.
One other thing. We don't base defense policy on today's threats. We try to predict future conflicts, and gear our equipment to that. Cold war based gear is on the way out, or being cut back. Commanche, Crusader, Virginia class subs, all cold war based systems. M-1 Abrams numbers are being cut, in favor of lighter, more deployable systems. F-22 is on oxygen, and should be killed, but the Air Force Fighter Mafia is selling out the service to save it.
Today's threat, of the terrorist in a rowboat, isn't 100% countered with equipment. Our passive defenses could use some work (armour at the waterline, anyone?), but we have had the firepower to counter the terrorist threat since 1918, with the adoption of the Ma Duce. However, we can't light up every boat that gets within 100 yards of our ships (In port, that is. Underway, heaven help you if you get that close). We have to find out which boat to light up, and that is the job of intel.
As we all know, intel in the Middle East has been lacking for near on 30 years, and I don't forsee it getting any better any time soon.
The threat posed to Navy surface ships is still the same, but the change is in the degree. We won't be facing a massed air attack from Backfire bombers gunning for our carriers. We will face the odd anti-ship missle fired by a few crazies, and current Russian anti-ship missles are scary, and being sold to anyone with enough cash. Just one hit from anything Russia has to offer would be fatal to anything smaller than a LHD, I imagine. CIWS can't keep up anymore.
Frankly, the line of sight kill capability with railguns is what interests me the most. If you can see it, you can kill it, out to an ungodly range.