JETA makes non-metallic bearings for rudder tubes that are specifically dimensioned to replace original equipment
of whatever make it may be. Using these bearings effectively ensures that there is no electrical
pathway from the rudder and its stock to the remainder of the boat (unless you make "improvements" that furnish a path).
That should preclude a repeat of the corrosion problems you say you have.
You say: "What we are trying to understand is how a modern foam rudder compares to the original solid cement one. After all, buying
a blue water boat
you pay for the peace of mind that comes with the solid build quality."
Compares in what way? Impact resistance? Ability to control the boat? Anticipated life span? As for the latter two there should be no perceptible difference. As for the first, when a rudder is damaged to the extent that it needs replacing it is not, usually, because the blade is damaged but because the stock is bent.
Since seafaring began, constructors have had to build "down to a cost" and skippers have had to live with that. You will find it easy enuff to live with if you understand the basics of the engineering that goes into each component of a boat - in this case the steering
arrangements - and therefore the limits to the abuse the components can take. Then you "skipper" so the components are not taken beyond their limits. Surely seafaring has to be the mother of the classic
Economics argument that it's impossible to satisfy "unlimited wants" with "scarce resources".
I think you will find that the original design was "adequate". You can - at cost - improve on it. The question becomes "is it worth it?". Repairs
are often worth their cost. "Improvements" are often not.