No mentioned argument ensures BB´s involvement in the final arc or his status as final villain, at least it does not for me.
Certainly not BB declaring it will be his age (when we know it won´t be, it´s Luffy´s) which is just an antagonist declaring their superiority, just like DD claiming he will rule the world, or Crocodile and so on.
Suddenly deducting a special role, or let´s say the most special role besides the protagonists, from that only does not really work, even if the moment BB says this, namely the end of WB´s era, is equally important to what he says.
From an individual perspective, there is without a doubt no character in the series that is more profilic than Blackbeard as an antagonist, i do not think i have to repeat the reasons, they have already been mentioned to death in this thread and others.
While i do think many people underestimate Akainu´s value maybe not as a character (he barely got any characterization beyond absolute justice terminator) but as a necessary adversary for Luffy to overcome, since he is the only guy who ultimately broke Luffy in the entire series and made him completely give up on himself, in a manga in which concepts like determination and the strength to believe in oneself are not only generic truism but at the heart of the entire story, BB´s value as a dark mirror to Luffy is not really up for debate, it´s big.
What i personally have a problem with, and this is up to Oda to make it compelling and credible if it comes to that, is BB´s eventual reason to still participate in the big war in which he has no real stakes in, unless Oda shifts BB´s role and aim from becoming PK to generally ruling the world with an Ancient Weapon like Crocodile wanted to. Because BB´s aim is without a doubt to become PK, unlike let´s say DD, his definition of becoming PK is not different to Luffy´s, get to Raftel, find OP. That´s why he is so happy when WB confirms the existence of One Piece, and it´s consistent with his insistence on the importance of dreams in Jaya.
But this also implies that BB will have no adversary role to play towards Luffy becoming PK since BB being still active in the final war and being Luffy´s last opponent to beat implies that the race for PK is over, that much is certain, since getting to Raftel and finding the One Piece is what makes you PK, and we pretty much have our confirmation that finding One Piece will inevitably lead to the revelation of the WG´s secrets, void century and stuff, and also ultimately to the confrontation with the WG, hence Luffy has to become PK before confronting the World Government.
Two Ds who have been built-up as ultimate rivals for the title not duking it out for the title does not sit well with me at all.
Of course there is another possibility, namely the World Government getting toppled without Luffy´s direct involvement in form of a fight or otherwise.
He fights BB for PK title, OP is getting revealed, others topple WG in the meantime with the knowledge that is revealed.
This would result in BB being Luffy´s final opponent, they still fight for the PK title, and WG is still getting beaten.
With this though, the only constant and consistent adversary of the entire series would basically gets sidelined, the people who have shaped the world as it is, have caused pretty much most pain in the entire world either directly or indirectly by looking away in pursuit of power, are either directly or indirectly involved with the majority of the arcs (Ancient Kingdom, Shichibukai, Marines, government agents), and quite frankly are the actual opposite to everything Luffy stands for, are not beaten by the protagonist but side characters with even less characterization and build-up than many individuals involved with the World Government.
Third possibility, Luffy fights BB two times, one time for PK title, then BB somehow gets stronger (or finds Pluton and can use it) and comes back, WG is getting beaten in the meantime and he will remain as representative of future chaotic times and clashes with Luffy.
How likely that is for a mangaka that more than anything else looks forward to drawing new stuff and has never really rehashed a main antagonist to become one again, i leave it in the air.
Nothing is certain at this point but if Oda makes BB the final antagonist, these are the hurdles he has to overcome, at least for me, for it to be credible and consistent and before i will say for certain it´s BB. If he of course completely disregards these points, then there is no limit to what he can do, but i know how i would evaluate such an outcome.
The one problem the WG has as a hurdle is the lack of an individual that stands out and represents the entirety of the World Government, which is a fair point. Like i said, i do think Akainu and his significance get underestimated, but he, first and foremost, does not stand out individually.
He is still someone viable to me because
Luffy has to beat him at one point, the significance of Ace´s death, and the emotional scar, symbolized by the actual scar Luffy showcases since the timeskip, coupled with being the only guy to actually have broken Luffy completely, so that he needed someone else to pick him up, is way too big to give Akainu to another character, and the best time, or heck the only time, in which such a match-up makes sense is in the end.
we already had an example of the main antagonist and fighter being separate from the actual leader of an organization, namely CP9. Granted, it´s on a much smaller scale, but the general setup can certainly be compared. Gorosei and Im being the ones to decide, just like Spandam did, Akainu being the executioner.