I apologize if this comes across as aggressive but why do people come into a thread just add nothing but negativity to it? Like, I don't care at all about Steve or Decidueye or something but that doesn't mean they don't have people that would be hype for them, nor should I walk in and say "your overrated bird pokemon sucks because there's already 9 pokemans." It's one thing to voice your complaints about it on a different thread but going on to a support thread and saying this is the equivalent of walking up to a woman minding her own business and saying "I have to say your dress looks like dog****." It's OK to think it but it's just plain rude to walk up on somebody else's parade just to take a dump on it, we're not 4Chan here.
It fosters discussion. Echo chambers aren't good, and it's important to know the opposing views. He went at it in a respectful manner, too. That kind of thing should be welcomed, it breathes life into discussion.
As for his points, I honestly agree that Fire Emblem shouldn't have as many characters as it currently has if we were only talking about how iconic or successful a franchise is. But that's never been the case in Smash Bros, really, or else we'd only have Mario characters, Pokémons, Link and Zelda, Wii Fit Trainer and the Miis. Just looking at pure numbers, Fire Emblem should be glad if it had anything besides Marth, Robin and Corrin, with Fire Emblem Heroes' Veronica as a newcomer*, but again, that's not the case.
First of all, I think it's unrealistic to expect fans of a character or franchise to be impartial when we are talking about what they want. Does, say, Isabelle, deserve to get into Smash before Captain Falcon? By most metrics she does, but does that mean that's something the fans want? If you were to ask an Isaac fan who "deserves" to get into Smash, if Isaac or Inkling, well, I am sure most would agree that Inkling deserves it more, but that doesn't mean they don't want Isaac more. Black Knight is a secondary antagonist of two of the least succesful Fire Emblem games, so I don't think anyone will say that Black Knight "deserves" to be in Smash. But if you like the character, you'll want them in Smash, of course.
And yeah, the oversaturation of Fire Emblem characters is a bit too much, but then again, Fire Emblem is a franchise that appeals through its characters. Especially nowadays, with FEH being a succesful gacha banking entirely on them. You can argue that it's in no small part due to Smash Bros making characters like Ike and Roy extremely popular, but that's irrelevant to the point that Fire Emblem has loads of characters to be used. And its Black Knight's merits as a character that make people want him in Smash, not that he deserves it. A character like King K. Rool was a glaring omission in Smash Bros based on his resume alone, so you could say that he deserved to be in, but nobody is arguing that Black Knight is on the same boat. People just like him, and want to play as him. He is easily based off Ike, who is the most popular Fire Emblem character thanks to his Smash Bros appearance, so you know Black Knight would also be well received as he'd play similarly. Most people agree on his kick-ass design, and he even fulfills the villain quota many think it's sorely lacking, especially on a franchise with so many characters. Does that mean he "deserves" it? No, but it's enough for a lot of us to want him.
As an aside, yeah, there are franchises that should have more characters. My most wanted is Midna. Twilight Princess is literally named after her and that game sold almost as much as the entire Fire Emblem franchise. Dixie Kong was the star of her own game (that, again, outsold any given Fire Emblem game) and co-star of others. Kirby... I don't know anything about it, but it's big and popular so surely there's characters in there that also "deserve" it. But that's the thing, Black Knight being in wouldn't be taking the spot of a more deserving character. He'd be a character that got in because he was the cool-looking antagonist of a supremely popular character, who didn't need much effort because their fighting styles are literally the same.
*I totally think she'd be rad and I'd like her a lot as DLC. She's easily Fire Emblem Heroes' most iconic character and the face of Nintendo mobile gaming.