I'm not "claiming" anything, that's a fact in the comics. That's how legacy characters work, which Black panther is - T'Challa is a legacy himself! Killmonger was temporarily BP in Priest comics.
The timing argument is a distraction, characters aren't given credit for being legacies for how long they wore the cape all they need is to wear the cape. T'Challa is a huge draw for the BP franchise, yes, but that doesn't invalidate the BP as a mantle itself. All Marvel needs to do is replace T'Challa as the next Black panther in a movie and that's all they need to placate the legacy for the franchise, in context of why they need to do it - because the actor playing T'Challa passed and Marvel's going to be on eggshells for years doing anything with T'Challa now. People cared about T;Challa because he had movies about him, just like any other character they'll be able to do that with his successor. It is Wakanda Forever, not T'Challa forever. T'Challa is a huge presence in the frnahciee but the comics are more then just him sitting in a room, it's a broad tapestry they've barely started mining and they've done a good job setting things up with Wakanda for the first movie. Dick's not getting that because circumstances in the movies haven't called for it yet, and the DCEU is still years from catching up to the MCU in legacy heroes. And they technically did pass the mantle to Robin in Nolan's movies. T'Challa's the exception because Boseman isn't here to play the character, that caught everyone off guard.