It is a problem though since the hunt for PK will pretty much remain without a credible rival for Luffy while it also reveals a problem for BB. He wants to become PK and thus make it his era, if Luffy becomes PK, BB pretty much stays an empty villain who failed to achieve his goal.
That's why Luffy and Blackbeard will fight for who gets to be crowned that, obviously. Blackbeard will be Luffy's greatest rival and antagonist to the end, just as he's been set up to be. There's no one else who can fill that role. These two will decide the fate of the world. The only problem is that some people have decided this needs to happen at Raftel, but that's not remotely true. It's clear by now that this is going to be bigger than Raftel, it'll involve the whole world. Blackbeard, the man who proclaimed the coming era to be his, the man who speaks of conquering the world, the only evil D, the dark version of Luffy, the man with the power to destroy the world, isn't going to be anticlimactically knocked out of the picture before the great end war happens.
The ruler of the age is not the one who conquers WG and Marines, it was Rock´s age, Roger´s age, WB´s age, and in all through each era the WG remained in power.
Blackbeard has outright said he wants to conquer the world. He's not just aiming to do what Whitebeard did, sitting passive on a throne. Or what Roger did, reach the end and then do nothing with it. The evidence goes against that being Teach's ambition. Saying that he will be Pirate King doesn't have an innate meaning because it means different things to different people. For Blackbeard, we don't yet know exactly what it means but if you assume it means holding a title and sitting on his ass you're likely to be surprised.
I wish it was confirmed, at this point it´s people wanting that to be the case though, so that they can feel justified in their opinion.
The evidence is clearly stacked. Speaking about conquering the world, having an interest in ancient history. All the signs point to it and that's what good predictions are, based in clear clues and evidence. Nothing will be truly confirmed until it happens but it's as close as it gets.
If the leader of the world holds teh pictures of BB and Luffy up, he puts them up as threats against himself from a superior position, not as people who will remain in the end, there is no causality in that statement.
If anything, it underlines the ultimate stance and level of the WG, they are above all, everyone else plays in their pond.
Of course he is, he's in the superior position now, but these are the two people that will eventually threaten him and his rule. Blackbeard has announced his intention to conquer the world and that the World Government can basically go fuck itself because it's HIS era. By the end the WG won't be the biggest fish in the pond anymore, they'll be knocked off their thrones as the ones who rose to the top in the new era decide the future. We had even more foreshadowing for this extremely recently with Doflamingo, if you recall: