To WG, Dragon is a bigger threat than pirates.
WG and Celestial Dragon were unaffected when OG Yonko were around and not much have changed today. Whereas, Dragon is starving them and toppling WG puppets causing Holy Knights to intervene so that’s why I believe he will have the highest bounty.
I was of the same opinion, I had Dragon at 6B, but I think now that Oda has drawn a clear parallel(s) between Akainu and Dragon.
Akainu and Dragon were clearly very close, they have probably known each other since Dragon was a Marine. Akainu is almost obsessive about this:
It is no coincidence that they:
1) have the same age (55);
2) they are both heads of the two organizations;
3) as said, clearly have a common past.
One of the reasons I thought Akainu would have 5B was that Dragon almost certainly had them, in fact.
Now with Akainu at 5 I think Oda might want to put them "close together", and therefore something like Akainu 5 and Dragon 5,1 or 5,2.