I disagree with that. Sabo is the thought to be dead brother of the protagonist who seems to be the second in command of Dragon. He seems more important than one of the Blackbeard Commanders.
I suppose it depends who you think will be more important in the long run, the revolutionaries or the Blackbeards.
I personally think the Blackbeards, with their position as final villainous crew, are more important to the story.
Espececially since I don't think Burgess will fight Sanji, meaning he won't even get a M3 fight EOS.
Any villain who appears as an EoS opponent to any of the main protagonists is going to be monumentally important, regardless of the protagonist.