There are a number of factors that can lead to mortality following a brain injury, which is why it's always so important to seek medical care when you're hurt. There are a number of factors that determine if your brain injury will lead to disability or death.
There are people critical of bike helmets because they say that they aren't protective enough. The reality is that they do a lot to protect people's heads in crashes. They lower the risk of traumatic brain injuries (TBIs) significantly.
Suffering a mild traumatic brain injury (TBI) is difficult; the fact that it's called "mild" doesn't represent the reality for many patients. Brain injuries present with various symptoms, and no two people are exactly alike.