That's kind of what I was hoping for. I don't see an issue with T'Challa dying off screen of something like cancer or a brain aneurysm. These are things that can suddenly fell anyone despite advanced science.
Plus the next movie would be significantly further in the future than the last. Black Panther took place a couple years before Endgame and Black Panther 2 would be years after the 5 year gap. So the Wandaka Outreach and whatever else they set up would have been around for like a decade or more (at least for the non-blip people). Whatever they do for the next movie, they could just handwave any succession issue (they figured something out after the mess with the blip, T'Challa set something up since he knew he might die, etc) or any other issues that might come up but they don't want to be the focus of the movie.
I just can't see a good reason for Disney to recast T'Challa since it's not like they can't tell a story without T'Challa. The next movie could easily be about T'Challa's legacy (so someone else taking up the mantel or a number of people suddenly taking up the mantel in honor of T'Challa). Or it could be about Shuri figuring out her place in the world after the death of her brother or dealing with someone or a bunch of people taking up the mantel as superheroes. It could even be about M'Baku (which I definitely wouldn't mind).
Also, people will be pissed if they recast while the people who are in favor of recasting shouldn't be too upset if they don't. So it's a pretty easy call not to.