I think the hard shell with just some padding is quite out dated. The reason why a shot to the side of the mask hurts so bad is due to several reasons.
The concept of a fiberglass shell with some padding between it and you puts the impact too close to your head. You/we need more of a 'crumple zone' between the impact and our bodies. Similar to getting shot at an armored car vs. body armor. Both do the same job (keep bullets out of your body) but the physical toll is vastly different.
The mask is open in the rear of our heads. Because of this, the mask moves and flexes with a shot to the ear area. Heck, the mask moves if you push it against your shoulder. A shell that completely wraps around your head wouldn't be able to push against your head since it would be a solid structure. The mask over our ears is basically a cantilever. Even if the mask had padding completely tight around your ear (bridge over your ear) would help - again, a helmet does this.
70 year old design. Why are masks open in back? Because that's how they've always been. Same reason why they're still fiberglass all these decades later.
Padding on the side of the head in a mask is too thin. Slim fitting masks are more appealing to players. Not so much the mask's fault, but they make the masks that still pass the mirror test. The mirror test shouldn't even be a factor. (but it always will)