So you say you're in the mood for a really dark worst-case scenario story?
Well, I'm not going to say this is a set-in-stone candidate. But I've had fleeting thoughts along these lines myself (i.e., "What if high school/college/pro football can't keep players from being concussed no matter what it does?"), and so I have to say I don't find this to be all that terribly implausible. In fact, I could see the end of football playing out very much like this.