I think it shows they were always planning on phasing out T'Challa and never had serious long term plans for the character in the first place.
Chad's death just sped everything up earlier than schedule and made it easier
Remember back in 2019/early 2020 it was rumored that Boseman was unhappy with Marvel about being replaced by Shurri.
You would think your lead actor passing and not recasting would cause a significant delay but BP2 was still going to make it's original release date that was set before Chad died even after he passed. That only changed when the Shurri actress got hurt on set. So they had to pause.
Marvel feel like they can't tell the story of Captain America 4 without the General Ross character, so recast, they feel like they could however tell the same story for BP2 even without T'Challa so didn't.
@Son_Of_God is probably right that the original script probably had Shurri as the main bp for half or most of the movie originally and T'Challa MIA for parts. If you look at this phase they've introduced/introducing female counter parts for the male Avengers. No reason why it wouldn't have been different here even if Chad was still alive