BB being the final villain is also headcanon.
Honestly if people actually accepted Shanks as a viable competitor for One Piece instead of always thinking he's mean to be jobbed by BB offscreen (even after this chapter my god), then its easy to just see how both can have separate arcs against Luffy, or even a 3 way fight for One Piece. Hell, maybe either of them survive past LT and are involved in the Final War.
There are so many other avenues to consider outside of the most basic one (i.e. BB = Final Villain)