I don't get why it's so hard to understand that fans of a good game want characters they like in their funny party game.
Three Houses is a game I enjoyed immensely and I'm fond of the characters. Therefore, I want Byleth or a house leader in the game. It's not for "Fire Emblem representation", it's for "Three Houses representation". They're not someone I'm dying to see added but they're someone I'd be very happy to see in.
I'm barely even a Fire Emblem fan. I've played most of Awakening, a large chunk of Echoes, and some of Path of Radiance and that's pretty much it. TH was the first one I've been absolutely obsessed with and it's because it's just that good. So the "you have seven reps!" argument is bull**** because, like, I don't. Sure I like the Awakening guys enough and have nostalgia for Marth, Roy, and Ike, but they're hardly "my" characters like Byleth or a house leader would be.
Also
this video is a pretty excellent breakdown of all the TH-specific things Byleth could bring to the table. Getting a character I like with a genuinely cool moveset like this would be awesome.