One Piece isn't real life, it's a fictional lighthearted story. Dreams are much more meaningful there than in reality, which is why Oda considers the loss of one a harsh punishment reserved to those that deserve it.
I wouldn't say it's as lighthearted as you say–in fact, I think Oda takes his work very seriously. Even though One Piece is humorous and fun in nature, I wouldn't say that the series is blithe.
And recycling a formula (somebody important to the protagonist dies, protagonist suffers a breakdown, protagonist matures and gets over it) is rewriting the Shonen formula? Maybe it is, but not in the positive way…
But it's not 'recycling a formula' without reason–he's letting Luffy experience grief and make a mature decision (albeit, remiss to his friends' desires, and to their chagrin). But growing from that makes someone stronger. I don't think Oda would have had Garp remark how much Roger hated losing his friends (and how it made him stronger) without intending for Luffy to lose some of his as well.
You talk like Luffy has never grew as a captain. He's not the same person he was back at the beginning of the series.
Also, this point has already been explored in the manga before when Luffy decided to recklessly punch a World Noble only to have his nakama attacked by Kizaru and taken away from him by Kuma. The following events made him realize how much he had to mature in order to achieve his dream.
When did Luffy realize he needed to 'mature' in order to achieve his dream? That's you projecting your thoughts onto him. Oda intentionally never includes Luffy's thoughts in 99% of all panels he appears in throughout the whole series. Luffy is still reckless, and he's still making claims that, frankly, I don't believe he can follow through with yet. He's grown in strength, sure. But maturity is a whole another boat. I'm not sure it's something he needs right now, but he will by the end of the series.
Duh, of course that was easy. That Marine Captain conveniently had the power to rust metal, a fact that was used as a plot point for Zoro to acquire a new sword in Thriller Bark. The chances of them ever meeting again is minimal at best.
I pointed to that example not because of that specific marine, but because of what happened. Do you remember reading that chapter for the first time? That moment was such a shock to me, and yet it only gets two panels. Zoro lost something essential to him like it was nothing. The rusting power was extremely convenient, but there are so many other ways for a sword to break. Especially in battle. Swords don't last forever; considering its heavy use, I would say that it's a miracle that the Wado Ichimonji has lasted so long.
Of course there will be more, but it doesn't mean that we NEED more.
Um, if Oda chooses to include them, then yeah, we need more. There's always a point or lesson he tries to teach to his readers with each one. None of the flashbacks or accounts of WG evils are superfluous.
Ace had no apparent dream and Whitebeard had already realized his. In order for Robin to die, she would need to achieve her dream first, which will most likely happen at the very end of the series. She might even die after that, but it would be too late for Luffy to give up on his dream considering the last poneglyph is supposedly located on Raftel.
But Ace did have a dream (or at least, he thought that he did)–at first, he wanted to be the Pirate King. Then he was defeated by Whitebeard. So his dream changed--he wanted to then make Whitebeard the Pirate King. He didn't accomplish either of the dreams that he stated, but the one he truly wanted was only realized at death: to be loved. That was what he wanted all along. His death was tragic because of that realization--he wanted to live because he finally realized his worth, but died soon thereafter.
Keep in mind that Whitebeard never realized his supposed dream either. He was cultivating Ace to become the Pirate King. His true dream, though, was to have a family. And he did that. In this series, there's a very large discrepancy between what characters say and what they actually want. It's cleverly done.
What is the Pirate King? Is it conquering all of the islands and reaching Raftel first? Or is it something else? According to Luffy, there's a difference. What's interesting is that we still don't know the extent of Luffy's dream–Oda hasn't revealed it yet.
And if what Rayleigh said is true, the real history is open for interpretation, which means that we need at least two people to reach two different conclusions (in this case, Luffy with an ability to hear the breath of all things similar to Roger and Robin's knowledge of poneglyph reading).
No. He didn't say that the history is open to interpretation–Rayleigh said that through the Straw Hats' adventure, they may come to a different conclusion with the information that they learn. They can choose to do something similar to the Roger Pirates (foregoing the evils of the true history and leaving it to someone else to solve), or do something completely different (like destroying the system). Roger "hearing the voice of all things" is how he could decipher the Poneglyphs; he even was able to write on one! So why couldn't Luffy?
They don't really need to die for that, though. If Luffy THINKS they are dead, then it works too. Oda uses death only when it's necessary for the plot.
But it would be necessary for the plot. I never said that all of his friends will die. But I think that after the trauma from Ace's death, Luffy would give up on his dream before he let all of his friends die. If one of them died because of him, in order to "protect everything," Luffy would give up on his dream. During the Marineford Arc, Luffy literally stated that he would save Ace in return for his own life. His friends are just as important to him, if not more so because they're all he has left.
So Oda will make Blackbeard kill a character just to add fuel to the fire? While not exactly purposeless, it would still feel like a waste of a good character.
I mean, Ace still had potential left in him and he died. But think of it like this: these people (good characters or not) are standing in the way of Blackbeard's own dream. But instead of just destroying their dreams, he's the type to kill his adversaries. Ergo, anti-Luffy.
He will become stronger in the New World, where he will be challenged all the time.
Strong enough to protect everything? Maybe in the end of the series. But he'll experience more loss before he can truly do so.
Just because you don't believe, doesn't mean that he can't.
While you may be able to place your faith in him, just listen to what he's saying. He's a 19-year old kid, and he's saying that he'll protect everything. Nevermind that this is a series where the villains are murderous pirates or governmental officials, but Luffy has been wrong before. Several times. I'm not going to say anything more to challenge your unyielding opinion, except for you to wait and see.
Well, I think you're the one underestimating Luffy while overestimating everyone in the New World. Luffy had the privilege to train under the First-Mate of the Pirate King and developed better than Rayleigh had expected. The SN and the Marines might have trained too, but not in the same way as Luffy. If everybody had trained like Luffy, then Luffy's growth would be nullified because nothing would have changed in the power scale (this goes more for the stronger Marines than for the more plot-relevant Marines like Smoker and Coby, not to mention the Supernovas, who need to be at least close to current Luffy's level to still be considered threaths).
Luffy has experience training under Rayleigh, but at the same time, he has none in the New World. In contrast, the Supernovas nearly all have been there for two years–alongside Smoker. That's where his disadvantage lies. The Supernovas are Luffy's peers and contenders; there's no way that Luffy is significantly stronger than all of them. I could see that happening in regard to a few, but definitely not most.