Oda’s greatest success as a writer may well be the fact that despite continuing screaming in our faces that Blackbeard is one of the most important characters in One Piece, there is always a large subset of fans who somehow think he’s essentially just an arc villain that will be dealt with before EoS.
Folk underestimate him as much in series as out.
I agree he's gonna be way more than just an arc villain but I think it makes much more sense and would be a much better narrative for Koby to be the one to take him down.
Allow him to genuinely earn the Hero title and not just given it based off lies and propaganda like Garp
Allow the readers to see him grow into the power house he is being hyped to become
Allow Koby to take down the dark reflection of Luffy rather than Luffy wiping out every big bad
Allow him to avenge Garp and rescue him from hachinosu even if you wanna say the strawhats help them out i still think Koby should be the one to finish the fight
Similar situation to Wano, yes Luffy had lots of help against Kaido until he grew strong enough to go the ones with him and eventually win, I think a similar situation will happen with Koby and BB