Rear radar also exists, Garmin Varia for example. I'd trust a helmet-mounted mirror more. The other thing about this overall vision for bicycles is that everyone has different needs. This sounds like a wishlist for a commuter e-bike, but it's not at all what road cyclists are looking for. And most of this does exist as an integrated package, from various ebike manufacturers. It's just expensive and heavy, and adds a lot of maintenance overhead for not much practical benefit.
AFAIK, it undermines the integrity of the helmet in a crash. Perhaps it's worth the trade-off, but also having some loose stick of metal flying around my head in a crash is conceptually unappealing.
I've never heard of the helmet integrity issue. Doesn't seem plausible with a sturdy commuter-grade helmet.
The way I think about safety equipment is that the most effective kind is the one that you always have with you, that doesn't run out of batteries.
Maybe the idea of having a mirror near your face is a concern, but hey, you're already in a bicycle crash, it's not going to be great either way. Wear some safety glasses for the flying debris. The best plan is to prevent that situation in the first place.
* "The strap and/or adhesive mounts could damage the helmet, especially if they’re installed less than perfectly."
* "the larger problem is the camera itself. A helmet is designed to have a surface without protrusions, and a camera represents a significant one. / ... "Anytime you add an external component to a helmet, you can change the performance of that helmet," says [Ian] Hall, [Consumer Product Safety Commission engineer]. "A helmet-mounted camera may become a projectile if it detaches in a crash, but a camera that remains attached could focus the forces applied to the rider’s head, increasing the risk of injury."
And from a Specialized helmet owner's manual: "Do not attach anything to your helmet, including mirrors or lights."
