I think the complaints about the tradition are really odd: 1) They severely underestimate the importance of tradition to societies, especially when it comes to choosing a leader. If they drop one tradition because they don't like the result of a challenge, then what comes next? What are the ramifications? It's directly applicable to what happened with the last US Presidential election. The GOP electors COULD have broken with tradition and not nominated Trump. But they knew that would hold massive consequences. 2) There were already people in Wakanda who both had doubts in T'Challa and who were inclined to support Killmonger's worldview (Specifically W'Kabi) So it wasn't a huge job to support him after he won the challenge.