Put an inflated rubber balloon on a kitchen table and drop a glass in the middle of it. It'll absorb the falling glasse's energy by diverting it sideways in the balloon. Now hold the balloon by the stem and drop the glass on it. Keep a dustpan and broom handy, because the glass will probably break when it hits the floor.
Eliminating the windshield from the supplemental restraint system is analogous to eliminating the table from the balloon experiment.
Yes, the windshield is a part of the SRS system. The airbag would still help without the windshield, just as the stem-held balloon would slow the glass a bit and divert it. But neither will have the effect it would without the barrier behind it to divert the energy in a controlled manner.