Taker never needed King of the Ring. He came in as a monstrous wrecking ball at Survivor Series in 1990, hell he feuded with some top stars in his first year. But the biggest thing, he beat Hulk Hogan for the WWF/E Title in one year. A heel went over Hogan, during Hulkamania. That cemented his spot in the upper card for his career. He has never needed a mid-card belt, let alone a mid-card push. He debuted and then steamrolled whoever he faced.
King of the Ring was used to groom the future World Champions. Look at Bret Hart, Steve Austin, Triple H, Kurt Angle, Edge. They needed a little bit of a boost to make that transition from mid-card talent, to the Main Event. Again, he did not need any boost from mid-card to Main Event. He basically debuted in the Main Event, and with his Deadman "powers" there was no reason to not believe it.
He doesn't need any other accomplishments. He is a legend in the WWE, and has worked his ass off for the past 20 years. I don't think the fact he didn't win a King of the Ring tournament is going to diminish his great list of accomplishments.