Unfortunately, given the way wrestlers from the 80s and 90s have died over the last few decades, I am not completely shocked to hear of Animal's passing, but it is still tragic nonetheless. Among the biggest factors in the premature deaths of wrestlers from this particular timeframe includes a drug culture that was prevalent in the business during that time, most notably anabolic steroids (which actually got WWE in hot water on two occasions, and even ended up in Federal court on criminal charges the first time). Although his cause of death has yet to be determined or disclosed, Road Warrior Animal, like his late tag team partner, was an admitted steroid user of that era, and past steroid use from that era may be a contributing factor here, especially if it turns out to be something like an enlarged heart.
As for Gale Sayers, he could've easily gotten in the 10,000 yard club if the surgical techniques of today existed in the 60s. To illustrate this point, Adrian Peterson tore his ACL (one of the same injuries that shortened Sayers's career) towards the end of the 2011 season and rushed for 2000 yards the very next season.