It’s a different situation from the Katakuri fight. Nobody saw the fifth emperor thing coming, but it’s now clear with hindsight that after WCI Oda wanted us to consider Luffy a different class of pirate from the YCs, symbolised with him beating Katakuri, Cracker and getting the Grand Fleet.
Whereas on Wano, he’s not beating Kaido so “cleanly”. Instead, Luffy’s needing nine Scabbards, four other Worst Gen, Yamato and who knows what else to beat Kaido. I don’t think the intention, after Wano, is for us to think of Luffy as already transcended Kaido, Big Mom and Shanks.
(Blackbeard is his own separate case, since he’s obviously going to get a new bounty soon as well)
Luffy’s bounty won’t go above Shanks’ until after the two meet- which was promised at the start of Wano to be soon. He might go into that meeting with a 4 billion bounty so they are basically equals, I think a 3 billiom bounty is more likely so Luffy can still be considered an underdog- or so readers can still be hyped up for Shanks. But Luffy going into a meeting with Shanks, already a billion above him, that has a bad effect. With something like that, we’re thinking “who gives a fuck about Shanks, Luffy’s completely surpassed him.”
And
@HA001 is right, Oda loves gorowase. Every Yonko pun (along with Roger and Whitebeard’s) is some sort of similar pun, Kaido’s bounty sort of means King of the Beasts, I think Big Mom‘s is old hag or something along those lines. When Roger was revealed as 5.5 billion, it became clear that at one point Luffy will go 5.6 billion to show he’s surpassed him.
So that can’t happen after Wano, because that’s obviously too soon, and it also means that Luffy’s bounty can’t be too high yet. Cause then a future raise seems shit. Going up 600 million was really impressive once, but not now that we’re going into the billions areas. 1.5 billion to 5 billion to 5.6 billion makes it seem like the last step is kind of unimpressive. 1.5 billion to 3 (or 4) billion to 5.6 billion is a steadier escalation.
And if it’s to three billion, there’s room for another quick Elbaf jump to 4 billion, if Elbaf is going to be an arc with a bounty after it (which it probably will be, for Usopp if nothing else).