They're not altogether useless. They keep the sun off your face and neck, which can be very useful in some climates. In Australia, where I live, a hat is now a compulsory part of every school child's uniform, and they are expected to be worn in the playground, because we have very high rates of skin cancer here. I don't know if that will translate into adult fashion or not. It might be too early to tell.

I suspect watches may diminish but will always be around because they signal status in a way that hats don't.