I'm not so sure about that though. Sabo isn't a direct competitor as a pirate. Luffy has directly exclaimed multiple times that he needs to defeat the Yonko (which includes Shanks and Blackbeard) to be PK. The first time he singles out Shanks.
I recall this being a very surprising moment in PH and one of the first times people entertained the idea of them fighting. Then the "excuse" became that because they are friendly, Luffy gets a pass.
Personally, I don't believe this is the case at all. But remember that when I say Shanks is an antagonist, I don't necessarily believe he's a bad guy, just complex and will be directly antagonistic to Luffy (gatekeeper, davy back fight, or straight up bad, an antagonist impedes on the protagonists progress, even if they "support" their ambitions).
Having Luffy essentially state he wants to fight him/defeat him really proves he is antagonistic as much we don't like to say. It does not have to be a massive change in his current character to achieve the same effect though, especially since we know very little still.
I guess we'll have to wait and see what he meant by that statement, but I believe it was just as simple as their crew joining the fight for it, whether that's a predestined antagonistic role or something else.
