Well said: I'll add that older glass boats, prior to the mid 70's were grossly overbuilt. Builders were used to thick wooden hulls and make the glass layout particularly thick and strong. Some boats were built with balsa core in those days can be 2" thick or more. Amazingly tough boats that are literally bullet proof. Installing a new thru hull for example needs to be done properly to keep water out.
Actually, A cored hull is much more expensive to build, should retain a larger portion of its strength over time since there is less fatique inducing flexure, and for any given weight, starts out stronger and stiffer.
That is why boats from the higher quality boat builders are built with cored hulls while the cheaper boats are not.
The fatigue issue is a serious one. Actual testing of older boat panels by an insurance industry study showed that fatique dramatically reduced the strength in older panels over time. But, and this is a big but, cored hulls only retain a higher portion of their strength if there is little or no delamination.
Balsa coring is popular because structurally it has better stiffness and sheer resistance than a similar weight foam coring. Properly installed, Balsa coring develops a strong and rot resistent bond with the skin.
In terms of long term durability of foam, there are a pretty wide range of foam core materials with equally divergent properties which makes it very hard to make a broad generality about the relative durability of foam coring.
In terms of build method, these days most cored boats are constructed using resin infusion or some other form of 'vaccuum bagging'. This results in a better bond between the core and the skins and in theory results in better protection from core damage. The key to preventing rot in a cored boats is avoiding penetrating the skins into the core or sealing any penetrations to avoid water intrusion.
A moisture meter can be used to check for problems. However it can be somewhat fooled by properly done repairs. Pay particular attention to areas around thru hulls.