All four are main characters. Having all three House Leaders present makes Byleth redundant on such a limited roster. But even beyond that, if it were a case where Edelgard and Dimitri were present but Claude was not, there would be a reasonable hypothetical explanation: Claude's canon Timeskip class being a Wyvern Master, a bow centric unit atop a fairly massive wyvern. I love Claude. He's easily my second favorite of the House Leaders. But considering he's over all the least popular of the three, the timeskip versions of the characters are by far the most popular variants, and a fully dismounted Claude neglects a very unique aspect about him while Edelgard's Armored Lord and Dimitri's High Lord classes are perfectly doable, Claude has plausible deniability. It's sad, but true. That's not to say he can't work, but he'd easily require the most work of the three by far.
Yes that's true, but then just make Claude an infantry archer fighter in Smash. The wyvern makes for a great recovery option, or maybe another Special where it blasts enemies with fire as it does in Three Hopes? Up B alone is great enough tho.
I mean, it doesn't have to be that deep, Claude doesn't even need to use the bow for all attacks. He has a sword in canon too, Lord is a class all House Leaders have access too, and there's a sword with his Crest, and he uses one in that one cutscenes with Nader.
Byleth doesn't need to be redundant in this case either. I imagine the Belmont's won't be there next game, and I'd rework Byleth to be based around the Sword of the Creator, and use a little magic. So Byleth sort of eases the loss of cutting both Robin (magic) and the Belmonts (ranged weapon for normals).
Also, Chrom and Lucina being Echo Fighters is not "beside the point." In fact, it perfectly illustrates the point. Awakening has so far been the only Fire Emblem game to receive more than one character, but only Robin is a fully unique character. You can't feasibly make any of the House Leaders Echo Fighters.
Then rework them and make Chrom and Lucina echoes of each other. No biggie. I couldn't justify not cutting Chrom while cutting Roy. But if we get a more unique Chrom and Lucina is his Echo, that works. Chrom could also be a semiclone of Ike and Marth, and Lucina a Echo to that.
Also the House Leaders could easily be based around the moves Byleth currently has with their weapons you know. Give Edelgard the Amyr attacks and Dimitri the Areadbahr attacks. That's already a good base, a little work is already done in this regard even on their movesets . Dimitri could even feasibly use some of the old moves of Corrin when they used the Dragon Fang lance.
It's honestly a good approach I think. Just gotta recycle some stuff.
Moveset potential isn't a good enough reason to do all of this when, again, you end up ignoring most of the franchise in favor of a single game in the process.
It's the most influential game however. And the most recent so far. New protagonists can always become DLC, and it's not like they couldn't cut the house leaders next game right.
This bit is willfully ignorant of what is actually going on in your examples. The Legend of Zelda keeps a fairly static cast, and at the time of Melee only had like six games. And again, Ganondorf and Young Link were last minute clones used as roster padding, and Zelda and Sheik were a package set at the time due to their switching mechanic. It's completely irrelevant to what you are suggesting here. The Donkey Kong Country example is even more asinine. The series keeps an ENTIRELY static cast for the most part. You don't have a new main protagonist in every ****ing Donkey Kong game.
And Pokémon is arguably an even more ridiculous point of comparison. It is literally the largest single media franchise on the planet. By all reasonable means, it is an outlier and should not be considered the norm. Any random person off the street, when asked, will know who Pikachu, Charizard, and Mewtwo are, more than likely. They've transcended the medium and became icons of popular culture itself. Fire Emblem, meanwhile, is still firmly in the realm of video games alone.
Pokemon maybe isn't the best example, of course they're iconic beyond gaming alone. DK and Zelda is different yeah, the cast tend to stick around. With Fire Emblem it doesn't. But that's another issue altogether, why should the likes of Robin, Roy and Corrin stay if they're no longer revelant ? I can excuse Lucina and Chrom due to popularity and Echo status, but the rest not so much.
Most of what you said here is entirely irrelevant (lmao, route length as a measurement of character popularity? Not to mention Dimitri's route in Hopes tends to get heavily criticized for spoiler reasons I won't get into). Here are some facts instead.
It's the best route though. I don't see the criticism. They tried to neuter both Dimitri and Edelgard from their original negative aspects however.
The in-game usage statistics taken from roughly six months after the game launched (in other words, the period of time where the most amount of people were playing the game) showed that Edelgard was the second most deployed unit in the game, second only to Dorothea, another member of (what do you know) the Black Eagles.
Dorothea as the most popular unit already proofs what am about to say, female characters in Fire Emblem tend to be the most popular. Hence Lyn is for example way more popular than Hector (and Eliwood). Generally speaking, if there are variations in character gender the female character is gonna be the more popular one. Corrin is a great example, but Byleth too.
Much about Edelgard being the most popular house leader came from marketing, she's hugely controversial as a character and many where put off by this. I don't quite expect this popularity to be lasting , not to this day.
But yeah it could be said, nonetheless I find it hard to believe she'd still be the most popular house leader.
In Heroes, whenever an A Hero Rises event takes place, usually the most requested free units are particularly meta defining and limited characters...with two exceptions: Edelgard and Dimitri, who had their base, brave, and fallen variants reach the top echelon of voting. This is a testament to both of their popularity levels. However, Edelgard always ranked considerably higher.
Finally, and most damningly, is Choose Your Legends. Some folks like to pretend this doesn't matter. However, it objectively does, as there are no fewer than two occasions where the results impacted things outside of Heroes. Namely, which Three Houses characters got Pop-up Parade figures (it's no coincidence that the six chosen were the House Leaders and then the most voted members of each respective house: Bernadetta, Felix, and Lysithea), and which formerly playable characters were excluded from Three Hopes (again, it's absolutely not a coincidence that Hanneman, Alois, Cyril, Anna, and Gilbert were the only ones cut when they were also the bottom five Three Houses characters on CYL). And in Choose Your Legends, Edelgard scored significantly higher in sheer vote count than Dimitri (who was second place) at a time when all then-currently-existing Three Houses characters were on the table for voting.
Every official source points to Edelgard being the most popular of the three. Being controversial does not diminish popularity...especially because if it did, it would affect Dimitri as well, who's also incredibly controversial.
Popular or not, I could indeed justify having Edelgard amongst Byleth and call it a day cause she's in a way the central antagonist of all the paths that aren't hers.
It's just , it wouldn't work. Byleth wouldn't use Amyr but would continue to use the Relics of Dimitri and Claude? That's even more weird than the original idea. So they way I want Byleth reworked gives way of a complete overhaul and also recycle the old moves but make full characters out of it. Dimitri could even get some moves of Corrin as I said, and maybe Claude could take some sword moves of Robin? Whilst Byleth takes the Thunder and Fire spells for Specials maybe. Edelgard could even inherit some stuff from Roy, like Flare Blade, and maybe her axe could have a mechanic that's similar to Roy's sweetspot but way stronger.
Don't say it's not practical, cause I think it's very much so.