Tsk tsk tsk
1) Don't multipost, it's against the rules
2) If you find it a mistake of Oda that's your opinion, however don't be so arrogant to start saying that your opinion equals the truth =/ especially not when, as I recall, you're the person who said that the whole of mankind are descendants from an alien race and just because you said it, it meant it was the truth and for which I eventually banned you because you didn't even provide a single argument (let alone a sensible one) and in the end the only thing you were able to do was insulting me. Everything what you just said was just plain childish: "why didn't he just do this or that" or "why didn't he let him lose it in some kind of grand battle" like it's as clear as day. That's the way how a child looks at the world.
3) A letdown? Everyone in this thread disagrees with you, you're the only one who is thinking that, which already proves it wasn't a letdown. There are always some weird individuals with twisted twirls in their heads who are never satisfied, but that doesn't take away that the general consensus is that it was far from a letdown, quite the opposite. I personally found it amazing to see how both Rayleigh and Whitebeard heard of Luffy briefly due to that incident while he was not even mentioned by name, and then to see both of them drawing the conclusion that it was Luffy Shanks was talking about when they met each other for the first time, like a puzzle that fitted perfectly. It are those small yet very subtle moments that make OP so great. It's just awesome to see Shanks talking about it so lightly when they asked "how did you lose your arm? What kind of terrifying monster was able to do that?" "Oh that? It's nothing, I just bet it on a new age".
Now Imagine that he wouldn't have lost his arm. Then no one would have noticed anything nor asked about Luffy. At most they would have asked where his strawhat is and then saying "I bet it on a new age" or "I gave it to someone promising" just sounds utterly stupid and they wouldn't think about it much. However having such an incredibly strong pirate as Shanks suddenly lose an arm in some far away peaceful region for the sake of a child has much more impact as both Rayleigh and Whitebeard, potentially the strongest pirates alive at the time, in fact already acknowledged Luffy when he was just a little kid solely because Shanks bet his arm on him, so what kind if person would that kid be then?
The entire mechanism behind these few short scenes is breathtaking, but it's beyond the comprehension of someone who thinks like a child. Now losing his arm in a so called grand battle now that would be a letdown. Why? Because that would be pretty much saying that Shanks doesn't have what it takes as if you lose an entire limb in such a battle, you're not cut out to be with the big boys. Losing a limb is something different then just getting some scars. If he would have lost his arm in a grand battle it would have made him look weaker than how he lost it in the manga. There he doesn't lose any kind of grandeur or respect, but he does it in the other case. But once again such things are way too complex for someone who sees everything like a child: extremely superficial. However that's one thing, but it's just plain arrogance if you then actually start proclaiming your individual, isolated vision as the truth while it's clear it's not.
4) Using Haki from a distance? That's what I mean with having the view of a child. Luffy was a hammer. That's even worse then someone who just can't swim as those at least can still float or keep their heads out of the water for a while. Luffy would start sinking at a very steady rate, so Shanks didn't have much time. If Shanks would have used Haki, the chance that he would have knocked out Luffy too would be high. Smart idea, knocking out the kid who's a hammer =/ Also if he would knock out the seaking, it would have fallen on Luffy and instantly kill him. Whether the seaking was still conscious or not wouldn't even matter as he was already attacking.
On top of that you need to realize that the Shanks from back then was not the Shanks as we currently know. Yes he was definitely a notorious pirate, however based on the manga he was not a Yonkou. Shanks was a pirate in training on Gol D Roger's ship, with other words he was not a "real" pirate yet. He became real one after Roger's death. So the period between that and saving Luffy is around 10 years. Mastering Haki is something that requires a lifetime. Luffy was a very special case as having a teacher like Rayleigh is extremely rare, most people learn by experience. So it would have taken a lot of time for Shanks to master Haki. His skills with Haki were definitely way weaker as they are now. And as said before he had to intercept the attack as even knocking out that seaking would have killed Luffy. Trying to save Luffy and using haki while intercepting the attack is far from simple, especially when you haven't mastered it.
So no matter how you look at it, it was not a mistake and even if you would look at it that way, the alternatives you proposed are even worse. But final note saying that it was a letdown or not is something completely subjective. So in such a case the "truth" is the general consensus, which is in this case clearly that it was not a letdown and that's just common sense. If you, as one person with a twisted vision, start proclaiming that what you say is the truth even though everyone disagrees with you, it means you're just an arrogant ignorant child.
Class dismissed U_U I have a canonization to attend to.