The design of the striker does not revolved around strength but rather stiffness.

A 4 mm wire can take the loading but won't add enough stiffness to the beam in the vertical plane. This is turn MAY allow the beam to bend to much (even with prebend) and raise the bending stresses in the beam to such a level that the beam may indeed crack or rupture. Without the beam taking the compression of the striker the whole setup fails and folds up downwards leaving the mast unsupported and destroying the whole craft in the proces.

So strength calculations alone are not sufficient one needs to include the stiffness portions as well. especially when looking at sail behaviour. A flexing mainbeam playes havoc with mainsail sheeting and trim.

That is the short version of my earlier posting

Wouter