I always thought Blackbeards cowardly act towards Whitebeard during Marineford (i.e. begging not to be hit) was strike one on him not being the true final villain
Strike two for me was the extreme reliance on broken devil fruits to say he's unstoppable
Strike three was when Whitebeard tells Blackbeard that he isn't the one to save the world via Roger's will. If you are someone who believes Luffy will become PK then save the world (i.e. final war), its very possible Blackbeard is either defeated at Laugh Tale or he's defeated before the final conflict is even over.
I personally have always seen Blackbeard as massive, endgame material, but not fundamentally the final breakthrough point which caps the series. This is also typical of a shonen story, where the hyped up endgame villain isn't the true endgame villain or antagonist.
Like that doesn't even factor in Akainu, Im, or even potentially Rocks if there is some plot twist there, but my money has always been on Shanks. Hes how this manga started, I feel he will be how it ends.