Your post hasn't convinced me, but you weren't really making an effort to do so anyway.
It's pretty clear that this isn't remotely some kind of gift but a product to buy. The product didn't appeal to you. It's fair to be mad about that. It's not fair to act entitled about it because it doesn't appeal to you. There's a huge difference in entitlement and disappointment. Disappointment is completely reasonable.
The fact that I don't want FE fans to get special treatment doesn't mean that I want them to be miserable. Pretty bizarre leap of logic there.
They're not getting remote special treatment, so your point doesn't work. Byleth had something called "the game actually being at the correct time to be considered" as well as the fact they had unique gameplay. So no, it makes sense why they were chosen. One can also argue it has promotional value that the others failed to have the right timing for. If the other games came out later, they may have had a chance. Likewise, for Corrin, since we're talking about promotion, they could've not been in but we'd still get a rep from an upcoming game anyway, since that was the first half of the reason(the other is the team convinced Sakurai based upon the moveset. If the moveset didn't work, we'd have gotten someone else anyway). In both cases, what overall worked out is the moveset factor, with timing also mattering, but less so.
And I won't bother to outright quote the rest, but showing empathy doesn't mean acting entitled too. It goes both ways. People have empathy for non-FE fans. But it's just a game anyway, and there's little to worry about. The second someone acts entitled they lose that empathy because they're being completely unfair in the situation.
Fans are entitled to nothing in Smash. Whatsoever. For Smash Fans, getting their cool thing is luck. But there's zero luck involved anyway, outside of a game being delayed/not releasing at the right time.
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This is also worth reading, and despite long, might give you an idea of why things are not that simple.
Besides, characters are only one part of representation. A lot of series has a ton of content, way outweighing FE by far. The Zelda series has way more content to begin with, despite 2 less characters as is. They also got a lot less major protagonists to draw from, so it's harder to properly get every Zelda choice in, especially when the recurring characters can often change design or have other issues(Tingle still has some Western hate, though less than before. Impa literally has no consistent design, and the latest one is not remotely usable at this time. Ganon is basically there as a boss and Final Smash. Midna with Wolf Link is viable, but also from an older game, so recency is taken into account. The rest of the choices often can have recency issues or he might not be willing to choose some over the others. Take the Champions; there's 4 on equal ground. If he can't choose one over the other, or all four, his choices become harder. We see Link updated to BOTW, though he doesn't use any Champion moves sadly. On the other hand, we lost the classic Adult Link too. So other than a 4th Link, there wasn't a way to win... and to be fair, I'd love Classic Link, so eh. That or a new variant of Dark Link with that moveset, I guess?)
I could go on, but every series is vastly different in how it works. FE is more like Pokemon, where it comes out with new major characters that all have viable movesets. A lot of other series don't have recency going for it, or moveset ideas he likes. It's worth noting he's a major fan of FE, so depending on how the DLC was chosen, that is, a list or not, a bit of bias is probably still part of it. And it's his game, so bias is understandable. I won't say people should be happy with that either. That's unfair. Not everybody will be satisfied with the roster. Brawl and 64 were the only two which heavily satisfied me. The rest less so(not to say I'm displeased either).
