Idk where you’re getting this idea they couldn’t protect other ships?
Even ESSM has enough range to protect other ships, particularly in convoy.
ESSM was designed as the point defense missile system. It's not good at crossing targets.
It has a 50km range. IF you are on the threat side of a convoy, that may help. If you're on the far side of the convoy and it gets mugged by SLCMs, you're hosed.
Problem is, detection range of a supersonic target basically on the water is less than 30km. Klub missiles are doing 1km/s, ESSMs are only slightly faster at ~1.3km/s. This means that there are situations where the ESSMs are absolutely incapable of making an intercept on a missile not targeted at the launching ship. Any intercept geometry that puts the ESSM into a tail chase, you're going to lose.
Back to that "threat on far side of convoy" scenario.
Best case:
ESSM ship is ~10km off the side of the convoy. Incoming missile is detected at ~25km from the ESSM ship, ~15km from merchant target, basically in a straight line from ESSM to target to incoming. ESSM needs to fly 10km just to get to the protected ship which takes ~7sec, and in that time the incoming has closed to 8km from the target. About 4sec later, the intercept is possible.
If that intercept fails, there is no chance of a second shot making an intercept.
There's intercept geometries where the defended ship is less than 8km from the radar horizon (and admittedly more than 10km from the escorting ship). You lose in that case.
Sure a single school house for a single super expensive radar does have its benefits, but we’re also seeing the downsides. Basic ships being more costly than they should be.
I didn't think I'd need to explain ESM passive detection and classification to a former Sailor. There is a
very large informational difference between "I detect 3x SPY6" and "I detect 1x SPY1, 1x SPS48, and 3x SPS49."
What's that first group? Hard to say, probably 2 Burke IIIs and a Ford-class, but could also be a Burke III, a Connie, and a Ford. Could even be a trio of Burke IIIs.
What's that second group? A Tico or early Burke, a Nimitz, and a question mark.