@FolhaS:
I think this is a point of view case, then.
You're looking at it as if Luffy was the starting point of everything, and while he's the main character, not everything in the series revolves directly around him. Oda planned the basis of his world when he created the series and then inserted Luffy in it.
Most of what you said is the reason why Luffy is the main character, though.
When the series starts Luffy seems just like some guy going on merry adventures and having quirky traits but as the series went along other characters started dropping hints about his similarities to Roger, that's part of the plot. Bear in mind that the strawhat connection was there since the beggining.
We, as the readers, are following Luffy because he's the most special person in his generation, the same way Roger was before him. This is pretty clear at this point.
Honestly, sometimes Japan has trouble making a protagonist actually starting from nothing.
I mean, Dragon Ball did it well, despite not being planned, and before that you had stuff like Tiger Mask, where the protagonist in an orphan that becomes the best wrestler because he trains like a mofo. But now? Naruto was a poor orphan until of course he was the son of the 4th hokage and one of the best kuniochi and was actually prophetized by frogs. Ichigo was just a random guy until of course he was the son of one very powerful captain. There were small hints to both of these examples but they pale in comparisson to the work OP has put in telling us that Luffy and Roger are similar.
I guess that in Oda's view for someone to conquer all those seas he has to check many specific boxes of skills.
All that being said, the only example I have as to why they're different is that Roger is more like a real-life pirate that Luffy, so I do get your point.
As a stand alone character he's bare bones, but I guess that makes him work as a mcguffin until the day we get a truly fleshed flashback with him. The audience can project him the way they want, like Luffy does. That's why everybody gets exited when he shows up.
Btw, I don't think Luffy has any trouble taking a kid on his ship if it warants that.
He took Chopper in, who was only 15, and that's in human mentality age. His biological age is actually younger. If Momo didn't have someone to take care of him or a will to go back to his home I'd bet Luffy would take him in a heart beat.
And Shanks' crew is filled with mooks. Sure, he seems to have a small, more distinct group of comrades but they're still surrounded by nobodies.
Luffy is the only captain in this series (outside of anime fillers) that has no mooks on his crew.
I don't mind Luffy being special. I just don't care about how the way he's special is often related to how he's like the previous Pirate King rather than standing out on his own. That is solely my personal preference though, it's just not my type of story trope and I don't consider that to be a legitimate problem for the series' quality. However, the straw hat connection is way too on the nose for me. I can stand Luffy being a D. I can stand Luffy being coincidentally related to/friends with Garp, Dragon, Ace, Sabo, and Shanks since childhood, especially since they never really helped him during his quest to become the Pirate King so much as only give birth to, train, and inspire Luffy as a child. I can stand Luffy having a similar personality to Roger, even if it implies that being the Pirate King means you should have the same type of dumb, reckless, prideful, and compassionate personality as Luffy and Roger. But the reason why I can accept all of this is because none of these things easily seal the deal for Luffy having the Pirate King title handed to him rather than needing to be earned. If anything, they often seem to attract more trouble around him than actually benefit him outside of how his personality inspires others.
But the straw hat being inherited as far back as Roger though? The guy whose greatest feat is Luffy's dream to replicate? It doesn't cross the line of feeling that Luffy is too advantageous in becoming Pirate King compared to his rivals, but it gets awfully close to it tonally/thematically. The Pirate King wore it. A Yonko wore it. And now a Pirate King wannabe is wearing it? All supposedly by coincidence? C'mon. I also feel that it somewhat spoils the reason why Luffy cares about it so much. It is his greatest treasure aside from his friends, and that treasure is not vain like wanting gold but shows how much he treasures bonds and promises as having real value. There was nothing hinted to be special behind it besides Shanks saying "that hat means a lot to me", and Shanks gave it to Luffy because he was a gutsy kid. At least, that's what the context was before we got all of the "I'm betting on the next generation" and "he said the same words Roger did" stuff (the former's fine, but the latter is falling into a repetitive pattern). So it feels almost similar to what happened when Naruto was built up to be a hopeless underdog that could prove hard work is more valuable than natural talent before we learned all of his special bloodline/reincarnation connections. This situation is nowhere near that bad since it's only a straw hat and Luffy was always sort of special (and to Naruto's credit he still had to give a lot of effort). But it also feels conflicting to me because the straw hat is actually a very valuable item after all as an artifact and almost seems to symbolically bless whoever wears it, or can only go to people with some kind of mysterious birthright rather than being shared through honest friendship/admiration.
The straw hat connection might have been there, but my point is that I find it VERY unnecessary, even if Luffy has to be propped up as being a special breed human rightfully suited to become Pirate King. It doesn't add anything to what we can already figure besides making Luffy look even more coincidentally special. Oda might actually make destiny a literal thing later on in the series with constant hinting towards that, but this is way too on the nose for my taste.
Chopper is a teenager. And biological age does not matter because of his Devil Fruit altering his anatomy and Oda placing no emphasis on reindeer lifespans in the story. Momo is eight years old. That is a big age gap. Add in how Shanks said that Luffy was too young to join his crew (Luffy was seven at the time) and how Luffy loves to emulate Shanks like asking Usopp to not bluff about putting your life on the line and not fighting back against Bellamy in the bar. Along with how Luffy and Ace promised each other that they would respective leave Foosha Village to become pirates when they became seventeen years old after Sabo died (aka, naive impetuous kid who went out to sea before he was ready and supposedly got killed). This isn't even the only case where Oda highlighted how it is unethical to bring children into a crew since that is why the Yorkie Pirates wanted Laboon to stay behind:
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So no, Luffy is never recruiting a kid into the crew that is not at least a teenager. Although I would love to see this dichotomy between Luffy/Shanks and Roger explored more when we finally see the latter in-depth later in the series.
Mooks or not (I don't even really remember seeing any faceless nobodies past chapter one, but maybe they appeared during scenes of the Red Hair Pirates chilling on random islands), it's the emphasis on the inner circle that I am referring to. He has have ten members as far as we know. They were emphasized during Marineford and recent cover stories. And Mihawk might as well count since Shanks and him hang out frequently, which makes eleven. Guess what the popular consensus is for the Straw Hats' final roster number? Eleven. How many pirates are in Blackbeard's crew? Eleven. This is not a coincidence. Luffy was inspired by Shanks and declared that he wanted a crew that could surpass him. So it's fitting that Luffy recruits the same amount of people into his crew to outpace the Red Hair Pirates, even though in world logic dictates this is strictly a coincidence. Oda just really likes how soccer teams have eleven players lol.