Ken Shamrock was one of the biggest stars in the early days of the UFC but he didn't use his status as an MMA star to help the sport but rather he used it to help only himself, parlaying his success at MMA into a professional wrestling career (which, ironically, was his original goal that he abandoned to pursue MMA).
Shamrock's run in pro wrestling accomplished little for the "World's Most Dangerous Man" and cost Shamrock much of his health; his injuries acquired during his WWE stint helped to make Shamrock a shell of the fighter he once was.
Shamrock left MMA in 1996 and returned in 2000, fighting in Pride. He only went 2-2 there, with his most notable fight for the organization being a split decision loss to Don Frye, a fight which is considered to be the "last hurrah" of both men.
After fighting in Pride, Shamrock returned to the UFC where he went 1-4 (with yet another Pride fight—a loss to Kazushi Sakuraba—squeezed in between the five UFC fights, making his Pride record 2-3). Shamrock's only UFC win in the Zuffa era was against the aged, mediocre Kimo Leopoldo. Three of his four losses were suffered in a feud against then light heavyweight champion Tito Ortiz. Shamrock' career suffered an inglorious descent after that...
What if Ken Shamrock had never left the sport that made him so famous?
After John McCain nearly destroyed the UFC, Shamrock still would've left for Japan to seek a greater fortune. He would've very likely stayed in Japan since he was a superstar there initially and his fame would only grow as the popularity of MMA in Japan skyrocketed along with the birth and growth of Pride.
Shamrock would even go on to compete in the famous Pride 2000 grand prix where he would meet Royce Gracie for a third time, finally managing to best the Brazilian in a 90 minute match in which Gracie's corner threw in the towel due to exhaustion.
Shamrock would defeat Don Frye in their epic match at Pride 19 and would then challenge Tito Ortiz for his light heavyweight championship at UFC 40. Ortiz would be awarded with an extremely controversial split decision victory; many believed the fight was fixed to protect the more marketable Ortiz.
The controversy over the decision only helps to add fuel to the fire of the Ortiz-Shamrock feud but by the time of their second fight Shamrock wasn't the man he used t be. He loses via TKO in the second and last fight to Ortiz.
Shamrock would eventually return to Japan for freakshow matches and an occassional professional wrestling match or two. While he wouldn't become rich, he wouldn't become practically bankrupt in this time line either.