Fire Emblem is just a victim of circumstance is all.
Marth debuts in Melee. Melee also happens to be developed in 13 months with no time for a lot of new characters, so clone characters are used liberally. Roy is introduced as a clone character with some unique attributes, as advertisement for the upcoming Fire Emblem game. Initially, neither of these characters were going to be part of the international release, due to Fire Emblems popularity only being in Japan, but then Sakurai realized it'd be stupid to cut content like that and included them anyways, thus introducing Fire Emblem to a greater audience.
Hoes are not mad.
Brawl comes around, and Roy is exchanged for Ike. Compared to Marth and Roy, Ike is beefy and wields a claymore instead of a normal sword. Despite his game's relative nicheness, people like Ike, as he fills a unique role of a big sword/weapon user in the cast and has some cool moves. In a way, it's similar to how Marth was differentiated from Link, as all these different sword users were very different in their style.
Hoes are not mad.
Smash 4 comes out shortly after Fire Emblem: Awakening, the series' most successful title that saved it from extinction, and as such gets Robin as a character. While there's many other characters the general population would've preferred, no one was really upset at this. Robin's moveset was unique, adding a "wizard" character to the cast instead of just being a normal swordsman. However, due to Lucina disguising herself as Marth in the game's story, Sakurai and team decide to make her a Marth alt costume, like with Olimar and Alph, but then realize that with just a small amount of effort, they can make her (and Dark Pit) unique characters. They do this, but a lot of people misunderstood it as "taking the slot" of a more deserved character. It doesn't help that Lucina is the second Marth clone in the series. She also has blue hair.
Hoes are a little mad.
Smash 4 DLC comes around, and Roy is reintroduced. Because he's a nostalgic character, though, he doesn't garner much animosity, though now there are three "Marths" in the game, when Wolf was still sorely missing from the cast. Like Lucina, some people erroneously see Roy as having taken the spot of a more substantial character. But what wasn't erroneously perceived was Corrin, who indeed was a new character, from a game that hadn't been internationally released yet, and was also rather divisive in its reception among the fanbase, unlike Awakening. This was softened with the simultaneous announcement of Bayonetta, but as a followup to Cloud, it certainly wasn't a thrilling announcement when much "bigger" characters were still in hot demand. Combined with Lucina and Roy, who are really just circumstantial, the oversaturation of Fire Emblem becomes infamous.
Hoes are mad.
Ultimate season comes around, and Chrom is unveiled explicitly as an Echo Fighter, and most people are okay with it. With the new understanding that Echo Fighters are not "full" fighters, there's no confusion about Chrom taking the place of, say, Isaac. However, as an Echo Fighter of Roy, there are now four "Marths" in the game, all by unique circumstances years apart from one another, but collected in one game removed from their original contexts, it appears ridiculous on the surface level. However, no "full" Fire Emblem character is introduced, and generally, most people are happy with the roster and new characters, so Chrom doesn't dampen anyone's feelings.
Hoes are no longer mad.
Ultimate DLC swings into gear, and Joker sets a high bar for whom to expect. Hero isn't viewed fondly by many outside of Japan, given that the series is only significant domestically, but that choice is balanced by Banjo & Kazooie, who were a long-awaited dream pick from the Melee days, and a character owned by Microsoft to boot. Terry was divisive, so many hoes were mad, but as a major third-party character and significant fighting game character with a fun moveset, and with one more character to go, people who were discontent simply looked forwards.
Enter Byleth, Schrodinger's Challenger. Simultaneously an obvious pick, by virtue of being the player character of a new Fire Emblem game, and a dubious pick, by virtue of not being a third party character. With the third party--or specifically, introducing new series with DLC characters--pattern set by the previous four characters, it was assumed by most that "obvious" choices like Pokemon and Fire Emblem weren't up for consideration. But, excusing that pattern, Byleth would seem to be an obvious choice. With big third party hopes from Capcom, Bandai Namco, or any other studio still on the line, however, it's definitely a character that was far below the bar that had been set by Joker and Banjo.
Hoes' status.
For what it's worth, unlike Corrin, Byleth has the decency to come from a game that's already out and loved, having become the most commercially successful game in the series in less than a year. Also, despite the choice of Byleth, Sakurai and co. are definitely aware of the sword-user issue, and have done what they can to make Byleth's moveset significantly more interesting, giving the character a variety of weapons to choose from that, were it not for the blue-green hair (why do so many Fire Emblem characters have hair like this) and style, would otherwise make the character indistinguishable from the Fire Emblem cast.
With a second Fighters Pass announced, with one more character than expected, the only really poor thing about Byleth's inclusion is that it came as the final character in the first pass, and one that was announced in such a clandestine way despite not being very remarkable from a "blow your mind" standpoint. It's the same thing that happened with Incineroar, Ken, and Isabelle. On their own, those characters are not particularly disliked or a problem, but as the final characters announced after the bar being set so high, they weren't particularly well-received.
I'd argue that, at the very least, Fire Emblem fans have their new character that's exciting and from a game they all seem to love, even if some might have preferred a different character from that game, and for those who don't care for Fire Emblem, the Fire Emblem character that many considered (and seem to have been proven true) to be inevitable is now out of the way, so no one should be sitting in anticipation for a new character announcement feeling any dread that we might get another bluish-haired sword user.
Unless Fighters Pass 2 has another Fire Emblem character, in which case we brace for the Hoepocalypse.