Oh my God how many times do I have to tell you, Byleth was chosen before the game even released. Three Houses being a legit good game had ZERO impact on the decision. And you're comment "if anyone would know that a game isn't gonna be a flop, it's going to be the developers working on the game at that moment" is nonsense because so many developers who work games rarely think "This game is going flop." Todd Howard & Bethesda thought that Fallout 76 was going to be a massive success. Cliffy B thought that Lawbreakers wasn't going to flop. PlatinumGames believed that Wonderful 101 wasn't going to bomb. Everyone who makes a game have the confidence that it will not flop, if you don't then why are you even bothering to make it?
You're obsess with the newest hottest, good for you. But you need to understand how many people aren't gonna view it as a great thing. Look at Playstation All-Star Battle Royal, they got the newest Dante in that game before DmC even released, it's the newest hottest that was coming out that so many people had fate in, and it's a laughing stock of a game and character choice. You'll never know if putting in this brand new character that people haven't had time to fully experience would work out or not, Byleth is the less interesting character in Three Houses and didn't even make it that high in the FE Heroes poll. Having Byleth come out a year later would've been for the best because, just like Rex and ARMS, people had time to fully experience the game & characters and welcome the idea of these characters getting into Smash Bros. Here you have barely have time to get invested in the character and potentially could've blown up in your face in the long run. Look at Corrin, look at how many people start to dislike his inclusion abroad despite how unique his moveset is. Corrin was super fresh and new at the time he released and aged terribly. History could've repeat itself so easily, it's the biggest gamble you can do. There was a reason why people love Joker's reveal, he came from a game that people had time to experience and get to know.
Also we already had a brand new fresh character who's game was just about to come out on the Switch added to Smash Bros, THE LUMINARY!!! Dragon Quest XI S was just about to come out and the main character from that game was about be put as the default character in Smash Bros. You get to play Dragon Quest XI S for the first time on Switch and have that character be in Smash Bros, but I guess if the game came out a year ago on other system you're old as crap.