Hi.
I do agree with what you say up to a point. The screen is yes an integral part of the full structure. Also the frontal area has to deform and absorb the inertia from impact. The screen can crack or break if the bonnet or other parts cause damage. A laminated screen is made up of a sandwich of at least two parts glass and one of a plastic sheet holding the sandwich together. Laminated screens are strong. Ideally the passenger cell should not deform on NCAP tests as the frontal area should absorb inertia. My issue is if the screen breaks in these tests would mean the structure is not stiff enough.
Jaguar iPace one of the newest and safest 5 star cars you can buy from a high end manufacturer
Screen breaks in both front impact and the Pole tests, I don't see that being marked down because the windscreen broke.
Comments from NCAP "The passenger compartment of the I-PACE remained stable in the frontal offset test."
it seems its your assumption that if the screen breaks that a bad thing, but if you look at the construction of many cars the screen overlaps a long way at the bottom, out in front of the safety cell for things like rain water run off into the scuttle.
In an accident the bonnet can push into the glass, components inside the engine bay can be pushed back making contact with the bottom of the glass. and as it is tempered glass it doesn't take a lot to cause the whole screen to shatter, hit a small area on a corner and the whole thing will go.
Safety Cells are not indestructible boxes, they have to dissipate any energy that the front crumple zones cannot and therefore are designed to flex and bend. If they didn't they would not dissipate the energy and it would just be transferred to the occupants. which means you can get a huge transfer of energy around the occupants. That shockwave travels through the windscreen and can easily shatter it.
For reference