Pretty good direct if I do say so myself.
This has got to be one of the dumbest fan rule-esque arguments that I've seen in a long, long time. Rathalos Armor is super iconic because a lot of times it's seen as the go-to look for the Hunter, it's the look seen in pretty much most of the promotional work; it's literally their most iconic look, and honestly a community symbol. Sure, Rathalos itself is the main flagship of MH; but that doesn't mean that certain Hunter appearances aren't iconic either.
I don't really get what you're trying to get at with that argument that somehow the armor set is iconic, but the Hunter themselves aren't; and that means that somehow the best way to represent MH is through costumes rather than a fighter. It's like arguing that all the Chief/Doomguy popularity originates in their outfits, and the characters themselves aren't iconic, so the best way to represent their series is through a costume (in that it literally makes 0 sense).
And even then, I don't know what you're talking about the "first armor set being more iconic than the hunter", especially because most of the time you decide the hunter's clothes "default" look, and most of the official starting armor sets differ slightly in each game anyways (though I may be wrong on this one, since it's been so long since I started a new file).
Again, MH is definitely focused on the "Monsters", but that doesn't really mean that the Hunter himself isn't an icon either. I mean they don't even start off trailers with monsters anymore, but usually with the hunter and the palico/felyne exploring first (both in World and Rise, along with a couple others).
If the complaint is that MH has no "default" look, like you yourself stated; they tend to go with rathalos armor set in most cases. Kirin armor is also pretty popular in certain crossovers, followed by some of the other flagships. If the complaint is that it's a (mostly) silent character with no defined personality, well we have gotten the Heroes from DQ, and Joker from P5 (who really isn't that silent in Smash), and much like those characters MH also displays a ton of their "personality" through animations. On top of that we have also gotten Byleth, who has much less personality literally. In terms of characters with customization appearances, we have gotten Robin and Corrin; and both of them went with their most popular default appearance; which the equivalent of that would be the Rathalos Armor set.
And the complaint of "they negotiated with the IP in some form before, and it didn't get in as a fighter then; so why would it now", doesn't really work only for MH either. They negotiated for Geno and Mallow spirits during base game, that doesn't mean that Geno has to remain a spirit through FP 2 (hopefully). We don't know what kinds of negotiations occur in the back. For all we know, Nintendo may have wanted to wait until a new mainline MH game came out on the Switch, before it gave them playable representation. Obviously, it's all hypothetical.
And yeah, I find it pretty annoying when people consider a character a "lock" when a game is announced; but most people don't do that. It's only natural a character see's more discussion due to game announcements, and there is nothing wrong with that. Because ultimately, there have always been characters that got into Smash due to recent games; it's a natural part of speculation, and you don't have to tell people to shut up about it.
If we discount the promotional appeal of a character, we may as well ignore other things that factor into Smash as well, so things like fan demand, shouldn't really matter either. But both those things definitely do in some aspects.
So literally all I see you doing in this post, is trying to find any counterarguments to the potential inclusion of Hunter, without really looking from a objective place and considering the things that Hunter does have going for him; which is massive Japanese popularity (both as a series and in terms of Smash), the biggest Capcom series currently, a potentially super unique play style with the likes of traps, felynes, and hunting styles, and multiple upcoming Switch games.
On the other hand, to be fair; let's consider the disadvantages. It's already represented in Smash in some form, and Monster Hunter has always been more about the Monsters than the player (which while being a weak reason to potentially prevent the Hunter from joining, still is a reason), middling international popularity (in terms of Smash).
So all in all, I think MH has a pretty fair shot, not abysmally low like you suggested and likewise not a lock either. I've always said in the Newcomer thread, that the biggest Capcom front runners are Dante and Monster Hunter, and they both have solid things that go in their favor.