This is way down the line – but I feel like Oda is laying the groundwork for the ending of the Yonko system at the end of the series. Two of the reasons are: changing generation, and relationships of the key players.
The current Yonkos are of the older generation, and they HATE each other. Yeah Whitebeard and Shanks seemed to have respect, but it keeps being pointed out that members of the Rocks Pirates hated and tried to kill each other – and 3 Yonkos were part of that crew. The 4th, Shanks, looked up to the man who defeated Rocks Pirates, Roger, as his role model while Blackbeard wants to fulfill his idol Rocks D. Xebec’s dream. So the Emepors do not get along, and of course it makes sense that they view any action of the others as a threat.
The New Generation is being lead by Luffy, Kid and Law.
They are going to take over as the key players, similar to how the current Yonkos built their strength and came into those roles over time. But their relationships are fundamentally different in several ways – they treat each other as rivals not enemies, and their goals seem to align / support each other (granted we don’t know Kid’s true goal yet, but Law’s requires cooperation with Luffy).
Essentially, the Yonkos have always been enemies often genuinely trying to kill each other whose goals are mutually exclusive or contradictory, whereas the New Generation are (even if they won’t admit it) friendly rivals who will help each other out because doing so doesn’t go against or undermine their own dreams.
With those differences, I don’t think we are going to have a “Law and Kid will be two new Yonkos” because that system will be done away with. Luffy will become Pirate King and they all WILL be powerful and influential Pirates, but it will be in a spoken or unspoken alliance (or begrudging friendship) where they have their homes and do what they want to when they want to in a more open and less adversarial way.