The Yonko would become the Sanko, because nobody else is strong enough to fill the void.
After Marineford the Whitebeard Pirates didn’t automatically become Yonko Big Mom’s crew would still lose their titles.
They also didn’t give it to the Blackbeard’s either until a year later, and even the Marineford Blackbeard’s shit all over the Kid and Heart Pirates.
Kid and Law don’t have anything that would let them deserve to be Emperors.
Individually, they are far too weak
Their crews are far too weak
They have no territory
They have no fleet
They have no influence
The Strawhats became Yonko’s because Luffy is strong, his crew, his crew is strong, his fleet is strong, they have territory.
Buggy became an Emperor because of his connections to other powerful figures, Mihawk is a monster, Croc is a genius, Buggy has an army and the threat that Cross Guild is directly causing to Marines.
Cross Guild are the weakest overall crew as a Yonko because it’s basically just Mihawk and Croc who are relevant fighters, but even with that Mihawk alone is enough to beat Kid and Law’s entire crews.
It would be like post-Marineford, the Gorosei would discuss who could possibly take over, the three potentials would be Katakuri, Kid and Law but they would wait to see if any were strong enough to actually fill the gap.