Honestly, Byleth is the biggest reason why people are complaining about Pyra/Mythra.
One of the biggest complaints about Byleth was that everybody thought the Fire Emblem rep stole the spot from Xenoblade. Obviously, that is not the case, but Fire Emblem is very over represented in Smash smash. If the LoZ franchise was given the same treatment as the Fire Emblem franchise, there would be 6 - 8 Links in the game.
Also, Byleth pushed people against anime sword fighters. Sephiroth was well received in smash and he is an anime sword fighter, but people were legit wanting him in Smash due to his importance in FF, and he has a lot of magic to mix up his moveset, so people gave him a pass. But the resentment against sword fighters wasn't removed, just repressed for Seph.