@Jabra:
I think this is a legit concern, but both Blackbeard and Akainu have abilities that are perfect to fight Luffy. Heat is still one of his weaknesses and Akainu is the heat king, while Blackbeard can disable powers and force Luffy into close combat. And that's us assuming BB is done powering up.
Sure, One Piece is not about power levels, and characters can pose a threat due to how their abilities interact, but I'm talking about another aspect, which is perceived narrative rank. One Piece is really successful because of how Oda explore narrative tropes to create tension and raise expectations.
Luffy beating Kaido, the "strongest creature in the world", one-on-one makes Luffy, to the reader's eyes, the new strongest creature in the world. You can't change that perception after it, everyone Luffy fights from then on, it's the other character that is trying to become the strongest, not Luffy. He's already the champion, he just needs to defend his title. To make anyone seem like a threat, Luffy would need to be defeated and then recover and fight again to reclaim the title for every arc from then on, limiting how threat is perceived by the readers.
Look at how people complain about Kaido being harmed by Scabbards, or how Luffy was about to go Gear 4th just to fight a perceived newcomer like Ulti. Those complains come from the readers' perception that the characters should be stronger than that. "He beat a Yonko commander! How is he struggling with a Tobi Roppo 'nobody'?" There will be complains just like that if Luffy, the top dog, has to struggle with anything bellow top contenders for his position.
By beating Kaido with help, on the other hand, Luffy is still not the "strongest creature in the world". He's stronger, but not the top dog. All threats ahead still feel like challenges to Luffy. By not being perceived as the top dog, Luffy still feels like the underdog challenging people stronger than him.
For a comparison, that's why Blackbeard wasn't allowed to beat Whitebeard one-on-one. If he had done that at the end of Marineford, it would feel really weird for him to run away from Akainu or not be the top dog of the world post-timeskip. But, since he had to cheat to win, we knew he was still growing and not ready for greater achievements.
The difference between Blackbeard and Luffy "cheating" is that Blackbeard is a scumbag seeking glory and not afraid to do dirty things, while Luffy is struggling the best he can to save others. The former cheats to reach the top, the later will "cheat" because there's no other way to save people otherwise.