Agreed, M'Baku to step in and carry the story would have been the best option.
Black Panther is an interesting film series. It's the only one what gets more than 50% of its BO from the US so you can kinda account for the "African-american surge" it might have versus a regular MCU film by shaving 10-15% off the BO (or, perhaps, consider it a "foreign office dislike of the leads" and add 10-15%???).
But Sam isn't T'Challa. Is Bucky even in CA4? It seems like a whole new universe carried off the back of a side character, more like Black Widow really, than a continuation of the Chris Evans Cap series. This looks waaaaaay better than Ant-man 3, Eternals, or The Marvels, for sure, about on par with GotG3 maybe, clocking in at $800 mill. But that ain't gonna cover the massive budget. Maybe Deadpool 3 will re-ignite marvel fandom, but even then its 8 more months till CA4.