I do agree that Kuma and King's lineage was more linked to biological reasons.
But here's the difference though. BB along with special lineage also has this tying him up with Void century war
Did kuma and king have this? No
Yes both luffy and BB are going for laugh tale but let me ask you something - who expected all elders to appear and cause havoc at egghead? None yet we got it
Who expected VP to make a speech talking about Void century and indirectly or directly (depending upon what he says next) frame WG for sea level rise? None of us expected it yet we got it.
Did we expect BB to get info on ancient weapons? No yet he has now.
What use Ancient weapons will come against battle with SHs? Why oda will even care to tie BB with ancient weapon plot if all BB is just a character to lose before laugh tale?
See, I am not denying the importance of IMU. Even I know what place he holds in the lore.
However, I am against the notion the notion of you people saying he will go down before raftel when we have hints saying othewise.