Something to chew on, but the characters we've gotten haven't necessarily gotten in because they're "important" or "relevant".
Daisy got in as an echo because she could be based on Peach and had a lot of requests. This is where Waluigi could not have happened, because he couldn't be a clone of somebody else.
We got Chrom, who is a clone of Roy. Chrom was added, specifically, because as Sakurai mentioned, he was popular among fans in Japan. We got Dark Samus because of ease of access to assets of Samus, and her popularity in America.
Like, I don't know. This fanbase talks all the time about how "popular" characters like Ashley and Mimikyu were locks for the game, and then complains when characters get in literally because of popularity.
I don't want to be a **** about it, and I love talking about Smash, but it's amazing how the discussions are almost entirely based not on logic or how characters would play, but more of, "This character is likely because I want them to be" or "This character deserves to be in because I want them to be playable". I can't take that seriously.
Deserving a spot is not only an argument that isn't rooted in anything factual (fictional characters aren't deserving of anything, because what they do is all because of somebody else), but it also doesn't substitute good gameplay, or gameplay that Sakurai just deals with because "Oh, well they're deserving. Looks like I have to add them".
Is the character FUN??? That's what should matter at the end of the day. Not how many copies their series has sold, not the color of the hair, and certainly not the weapon they use. Honestly, it wouldn't surprise me if Fire Emblem got more characters because their movesets are just...fun. They're basic. People like basic and fun movesets.
As some have said, despite the amount of sword users in the game, none of them can be played the same way. They're all different, even in small ways. The only reason fans complain about one type of character is, spoiler alert, because their favorite isn't in. If they get their particular choice, they won't give a **** anymore. Example) Me. I got everything I wanted, but still would love to see Geno because he'd be fun as hell.
Also, if the argument is that a series "doesn't deserve" another character, then there's not much I can say. Slots aren't a thing. Sakurai makes this game to satisfy the most people he can, and he felt he could do that by adding Chrom. Adding Chrom over your character of choice didn't actually happen, because he didn't pick between Chrom and your character. He picked Chrom because he took little time, and a lot of people wanted him.