So basically... 'If you don't think the way I do, you're not a real fan'.
That is terrible logic, not everyone is going to accept the lesser version of their favorite character being playable just to have them playable. People still complain about Ganondorf still carrying much of Captain Falcon's DNA even with the changes he has gotten, I still see many Dark Samus fans upset that she lost many of her unique abilities from being an AT, not even going by the Prime games themselves, and so on and so forth. Saying they don't count because 'well their character is playable' is ignoring what they feel and what they want out of their character, they are no more or less fans of the character just because they don't want to make concessions for them.
Also, your post ignores the major problem with Black Knight that Opossum brought up: Black Knight being an Echo would be a complete mess of a character. We wouldn't be talking Dark Samus levels of concessions, we'd be talking an entire character having to give up much of their identity to even work as many of the moves Ike has don't fit on Black Knight whether animation-wise or character-wise, and since we're not talking semi-clone levels of changes you're basically asking for a frankenstein mess of a character who clips with every move he does and lacks any of the presence he has in the FE games. I'm not sure many fans would be happy with their character being a mess to look at or play with just to check off 'well, at least they're in the game'.
Finally, getting to the point of 'well they can evolve with future games', there's a few problems with that mindset as well. Firstly, you're asking for someone to accept a poor man's version of their favorite character in the hopes it gets better in a future game, one that probably wouldn't happen for years considering Smash's release schedule. Secondly, there's no guarantee that the characters will change substantially in the first place. Young Link didn't change much between Melee and Ultimate, with the vast majority of his changes functionally being updates for the difference in engines, and similarly Lucina and Dark Pit didn't differentiate much between Smash 4 and Ultimate. You could basically be stuck with that version of the character permanently, with no guarantee that the following games would bother to go back and change them. Finally, this is assuming that they remain in the series after that point, lets not pretend that we know every character is going to return in every subsequent game, even Sakurai is unsure of that, so the concession of 'well, they'll change in the next game' is not a certainty and they could potentially be left with a version of a character that they don't like in that one game.
In the end, you're trying to push that your idea is the only way to go about this situation when there's a number of problems that can easily deter someone from not accepting it. Not everyone is fine with their character not being a better interpretation of themselves, especially when the interpretation you're asking for is likely to be extremely flawed in execution, and they could be stuck with that version due to the uncertain future that Smash games have. Not everyone has to accept that direction nor should they just to be considered 'real fans'.