one thing that I like to note with Ike's changes is his entrance animation and his idle animation, his entrance animation has his feet on the ground which could be seen as more suitable for the black knight (I don't think you could have BK entering with Ike's brawl and 4 entrance), and his idle animation has him standing straighter, meaning that if they wanted to make BK an echo, they would only need to change where his sword arm is, I think you could even make an argument in the case of a semi clone, if he is using Ike as a base, they may want to change a few things about Ike to make his options more reasonable to port over, rather than virtually start from scratch eliminating the purpose of a semi clone, and allowing more parallels between the characters.
the voice argument is also not without merit, though you can also note that this is the first time since brawl that Ike had a new (and available) voice actor for the new game, I think he is voiced by greg chung (probably spelt wrong), regardless, I think it would not be unreasonable to expect new victory clips for other fire emblem characters ("I never much cared for kings" against marth pls) if they are going this far out of their way though, otherwise I would see that as a waste of resources.
edit: regarding Ike's entrance animation, all that would need to be done is once again change the position of BK's sword arm, it is a rework regardless, but not near as punishing as as repurposing his 4 animations.