Bugger what gunnery experts want, the armchair Admirals and retired-but never-really-were-involved on Arrse know way better! Even when they proclaim to have no such knowledge....I am still limited in what I can discuss, but the 57mm will do all the tasks set out in the requirements to an adequate level.
If those requirements are correct is a slightly different question to why not a bigger gun.
To answer the "why not a bigger gun?"
A significant length of time was spent, by gunnery experts, looking at what weapon system would fulfil the requirements best, with some excellent analysis of both the physical effect of the weapons involved and the maintainability and so forth. The 57mm was the most effective, given the constraints of budget, time, training and so forth.
It also does all the jobs asked.
"Was the spec right?"
Unless you have seen the full requirements set (and I doubt you have), you don't know. I think it reflects a fair stab at what we want this platform to do in the future. ETA and that is all it will ever be...
A little knowledge is very dangerous and doubly so when not sighted on the context nor wider issues and some people would do well to accept that.
Time to leave this thread.