@Esc is spitting. Yamato's chances dropped even lower from these spoilers
1. Last chapter we're told that Yamato's freedom was stolen from her by the handcuffs that she's had on for 20 years. Luffy's like, "I'll just take them off" and Yamato goes on about how it's not that simple. You'd think that if she was crew material, Luffy taking off her cuffs would be a significant interaction between the two, Luffy finally freeing her of her hell on Onigashima. Something that would happen further down the line after the two had bonded and for it to serve as a significant straw hat interaction between captain and future crew mate.
That's what you'd think...But nope. Very next chapter they're off. And it's not a big deal, it's literally made into a gag and is just a small addition in the chapter that's heavily outshined by everything with the scabbards and Kanjuro's defeat. No straw hat is getting "fodder" treatment like that.
2. Translation by sandman Luffy: "This decisive battle should be started by them (Red Scabbards), not by us (Luffy and Yamato) !!"
Yamato is someone who just told Luffy her story about being locked up and and abused by Kaido. About how much she loves Oden and that she is in fact "Kozuki Oden". What does Luffy do? Not give a shit. 2 chapters later, when Yamato wants to attack Kaido and have a big moment, Luffy pulls her back and tells her that that's the scabbards moment, not hers. Despite the fact that Kaido is her father and her whole story she just told him. And this is something very natural. Obviously Luffy, and us the readers
should care more about the scabbards since they're the true inheritors of Oden's will and have been developed over the past few hundred chapters and further in a backstory. They weren't simply introduced out of nowhere in the last couple of chapters. But with Luffy outright stating that this is the scabbard's moment, not Yamato's or anyone else, very clearly sets out a hierarchy of importance. And there's no way a straw hat is gonna play second fiddle in a plot revolving around their own family, dream and backstory.
3.
Luffy's given Yamato a nickname. Something he's never done for a single one of his crewmates IIRC. And we know that has significance:
Whilst it's not hard proof that she's not joining, it certainly not a good thing that Luffy doesn't even care to remember her name. She's another "Hammock", "Cabbage", "Jaggy".