From a writing perspective, would it be more impactful to declare them as emperors now, after defeating a yonko, or way down the line after conquering some island nobody gives a fuck about?
From a writing perspective, no new Yonko should be declared as the current Yonko fall and the OP world is thrown into complete upheaval. The existence of the Yonko is a result of the existence of the balance of power, when the entire purpose of Luffy’s generation of Pirates being the “Worst Generation” is that they are a group who has been uprooting the power balance and the old systems of power.
Declaring three of them to be new Yonko not only makes zero sense from the criteria Oda has established for the title, but it also goes thematically against every aspect of the Worst Generation that Oda has been illustrating in his own mediocre way.
But I guess it was really necessary to introduce a gag news source 80% of the way through the OP story even though news circulation was never something important to the OP plot until it came time for Oda to force the feeling of Luffy becoming a great pirate by throwing the Yonko title at him lol.