I understand the OP's sentiment, but as a rider we should always be paying attention for other users, especially those in front of us. That hiker could easily be biker with a different skill level or first time rider on a trail checking out a segment, etc. which means a marked trail designation would not solve the situation you describe.
I personally tend to follow the same etiquette I use when skiing, I am responsible for the person in front of me.
Bombing down a trail is super fun and desirable but that also increases your risk of an accident be it the result of a trail feature, stick in the spokes, tree, missed turn or other trail user. Best way to mitigate, is to be aware yourself and prepared.
So perhaps signage on trails should remind riders, hikers, users in general to be aware of other users on the trail, which they kind of do already.