It's not headcanon. WB is PK potential. He has the strongest Paramecia devil's fruit, which encompasses both Law's as well as Kid's DFs. His Haki prowess is top-tier—he excels in all 3 forms of Haki. He's also got top-rated raw fighting states and physical stats.
Kid has an inferior fruit to WBs. Average raw fighting skills. Excellent physical stats. Has no portrayal points for Haki besides having all 3 types of it.
Law has a fruit that is nearly as OP as GGnM. Below-average raw fighting skills. Excellent physical stats—not as good as Kid. Average Haki stats with no CoC, which automatically eliminates him from the race.
WB has the best of both their qualities and more—both of them have no hype as PK-level potentials whatsoever. At best, they're similar to the contemporary Yonko. I doubt they'd even surpass the Yonko, forget about WB.
There is no such thing as "PK potential" which is referring to reaching "PK level". Unless that is your categorization of the level of Roger/Newgate/Garp/Shiki/Sengoku/Rayleigh all stood at...then sure I'll address the post with that in mind. But if it's for some imaginary level where Roger/Newgate were the untouchables then yea we might as well stop since that's just headcanon 101.
Their Ages & One Piece ages in general:
Eustass Kid - 23
Trafalgar Law - 26
When do people generally reach their peaks in one piece? Late 30s - early 40s. Whitebeard stated to have started his prime at 38.
For many their bodies don't even stop growing until their 40s, Garp being a prime example of this. As he got bigger physically from God Valley to his confrontation with Chinjao.
Kid is about 15-17 years away from his prime.
Law is about 12-14 years away from his prime.
And this is the lowest prediction of when they'd stop growing. Hell if we simply look at what Oda drew of Luffy's older age, his body doesn't even stop developing into his 40s, where he's a tank by the time he's 60 compared to his 40.
Age for Age accomplishment:
Whitebeard up until the age of 36 was an underling as far as we know. Whether it's the little crew he was part of without the mustache or whether it was under Rocks (maybe they were both the same who knows). He was the Jack (28) /Smoothie (34)/ Devon (34) equivalent, aka strong commanders who have not yet reached their peak, while being under a top tier. (Not saying they're gonna be as strong as Newgate, simply pointing out his role).
By the age of 23 Kid has:
- Challenged Red Hair Pirates and lost his arm
- Went against BMP and got what he wanted
- Went against BM & Kaido just like Luffy
- Has a solid crew with an underling like Killer who has also top tier potential akin to Beckman.
- Boasts a bounty of 3 billion
By the age of 26 Law has:
- Became a Warlord and left on his own
- Took part in taking down a Yonko just like Kid & Luffy.
- Has the 3rd best crew of his generation after Luffy & Kid
- Boasts a bounty of 3 billion
Based on what we know, they have factually accomplished FAR more than Newgate did at a much lesser age. No different than Luffy accomplishing much more than Roger at a lesser age.
Devil Fruit comparison:
If you reread the SBS Oda states that Gura Gura it's "considered to be" or "Said to be" the most powerful paramecia. That is the reputation the fruit holds within the One Piece world. That is not Oda claiming "Gura gura is the strongest paramecia", no it's simply stating it's reputation within the one piece world. And reputation in the one piece world is not something that can be taken as 100% factuality.
Gura Gura has more raw destructive power, yes, that doesn't equate to it being a better fruit for fighting than Law's ope ope no mi or Kid's jiki jiki no mi. Both fruits have more diversity in how the user can fight with them than the Gura Gura.
It's a lazy argument imo to say fruit with more raw destructive power = more potential when it comes to being the superior fighter, compared to other fruits that are much more diverse.
Physical comparison:
No point in any comparison, Whitebeard is physically the strongest human male to exist in One Piece world.
Haki comparison:
Whitebeard at the age of 70 showed great CoO skills with the sleeping and kicking Ace's ass.
Whitebeard into his late 40s showed great ACoC skill with having an equal opening clash with Roger.
These two at the ages of 23 & 26 have yet to display that sort of Haki, with most of their attention being focused on their devil fruit mastery for now.
However, what is Haki power based on? Willpower and ambition. Kid's willpower rivals that of Luffy's. Law's is up there as well. Their ambition also rivals that of Luffy's. Despite knowing Luffy has a better crew and all the advantages in their race to PK, they're not backing down. They have also never bent the knee to anybody, unlike Whitebeard who has to Xebec. He also never expressed any ambition rivaling theirs.
So their Haki peak potential is higher than Newgate's. Is it on his level right now? Oh heavens no.
Raw Fighting Skills:
Both Law & Kid had displayed more skill in just raw fighting than Whitebeard has. Who's skills have consisted of basic punch and generic swings of the glaive.
Kid has displayed wrestling techniques. Law has displayed CQC swordsmanship techniques along with stated to be skilled in other combat styles.
Hype for the future:
- The age for age comparison already serves as better hype than Whitebeard's.
- The first time the three are gathered together, Oda intiates a competitive nature between the three.
- Which he later then continues to build upon same arc.
- Even has Rayleigh comment on the performance of all 3 of them.
- And how does he end their meeting on Sabody? On a competition for the One Piece between the three, and buildup for eventual meet up in the New World.
- Coming in to New World, who have the highest bounties from the supernovas? Law & Kid. Then Law & Luffy get the same bounties.
- Then in parallel to Luffy-Law forming an alliance to aim at a Yono, Kid tries to form his own alliance to aim at a Yonko.
- Goes out of his way to add in competition between Kid & Luffy, where they are actively trying to out do each other and actually pushing each other to their physical limits.
- Then both are against competing to destroy the fodder ship pre-Raid with their special attacks.
- Rooftop fight starts, Oda again adding a rivalry spirit between the three to out do each other:
- Then Luffy with fuck ton of help + restarts takes out Kaido eventually, while Law & Kid with sum help from the bombs take out Big Mom.
- Both end up with a 3 billion bounty for what they did. While of course Luffy being recognized as the superior by becoming a Yonko.
- Despite knowing how strong Luffy is, neither Law or Kid backing down from competing against him. In fact Law wants to compete on an even playing field.
So two individuals from the same generation who Oda constantly throughout the series highlights as being rivals of Luffy, who are actively competing against Luffy are not "PK potential"... But the guy who never competed against Roger until he was in his 40s, is PK potential.. You might as well as Luffy isn't PK potential either.
If the argument is they're not on Luffy's level thus not PK potential, then that's just a disingenious argument. Since Whitebeard never became Roger's "equal" until Roger became sick. And as pirates Whitebeard was never a rival to Roger until he finally got Oden and even then Roger pirates were FAR superior.
But yes, Law & Kid in their peaks during 40-60 will be leaving a 40-60 year old Newgate in the dust. This entire young generation of pirates & marines is surpassing the golden era. These two won't be the only ones leaving Newgate in the dust. (And I'm not even including people like Dragon/Blackbeard/etc from the mid generation who also have the potential to be superior to the likes of Newgate when it comes their time to showcase their prowess).