Yeah and Oda decided to spoil it in an SBS as a joke...
Don't tell me that your only argument against Gear 5 is because Oda mentioned it in an SBS?
Oda did not spoil it as a joke, he pulled a joke to say that Gear 5 would happen against the strongest opponent (Kaido). That's different. Though that's not even a reason to believe Gear 5 is happening in the first place.
Why don't we use actual arguments instead?
Luffy currently uses basal form, gear 2, gear 3 and gear 4.
- Gear 2 and gear 3 he already uses them against Kaido and combined with aCoC, they are very strong assets, although they didn't take down Kaido yet in 1037.
- Gear 4 he used it mainly to infuse his CoA and fight opponents that required only CoA to be taken down (Doffy, Kata). He also used it a combined gear 4 + aCoC once against Kaido (along with Yamato's attack). In gear 4, Luffy coats his body with CoA, then he blows his muscles with air, allowing him to expand by remaining hard (hard but still rubbery).
Now, is Luffy going to pull a new gear 4 form to take down Kaido, an opponent that was clearly shown that gear 4 was not sufficient (Luffy got sent to Udon)? I don't think so. Gear 4 always relies on CoA, a haki form that Luffy doesn't need to fight against Kaido. It's a waste of haki and Rayleigh himself told him that it was a very exhausting form.
Also, Luffy does not only need a new haki form, he needs to use his devil fruit accordingly and gear 4 is a stupid gear to use aCoC.
Luffy needs aCoC and a gear form that does not rely on CoA. Also, Luffy is fighting a strong opponent and he has pulled new gears against opponents that required less than that.
A new gear form "Gear 5" is henceforth very likely to happen against Kaido.
Beside that, we also had Law's and Kidd's awakening, which kinda foreshadow Luffy's. It'd be kinda weird narratively speaking if Oda awakens Law and Kidd but not Luffy, knowing the three of them are rivals and are trying to surpass each other.
Gear 5 may involve awakening, or at least, an initial form of it.
Finally, we also had some lore about a devil fruit that is legendary for the Gorousei. It may also be that Luffy's gear 5 (awakening) is going to be related to that and that he actually is not rubber but that's a less likely option.