There were some arguments prior to the confirmation about how Dark Samus wouldn't work as an Echo because her animations and moveset should be more unique. The animation work is probably better than any Echo so far, so that's a privilege, and many people who had doubts have come around due to how good she looks, but others are still liable to be disappointed by the moveset. But that's not what matters most. Dark Samus could have a totally unique moveset, but if she's not smooth or fun to play, then what's the point? There's a Dark Samus mod in Brawl that I've seen with a unique moveset, but it looks janky with rough animations, bad modelling, and stiff moves. It's a nice novelty, but I imagine playing it for an extended period of time would just make me wish that the character was as smooth as the others more than anything.
Dark Samus's presentation in Ultimate is better than any other Echo Fighter we've seen, and she's programmed by Sakurai and team, so she's going to be just as smooth and fun as any other character. Whether or not she has her own moves, it'll feel good to play, instead of having her own moves and feeling awful. That's what matters most at the end of the day.
When I voted in the ballot for Dark Samus, I said something along the lines of "Her Assist Trophy already shows that she could be a fun unique fighter." This was back when Sakurai had shot down Ridley hard, so I figured if Metroid was to get any new full rep, it would be Dark Samus. But since we finally got Ridley, I didn't expect Dark Samus to be a full character, because that'd go beyond reasonable expectations for Metroid representation. I still didn't fully expect Dark Samus even with all the evidence, because it just seemed too late for the character to make it into the series (today is the 11th anniversary of the character's last appearance).
However, will Dark Samus appear in future games? It'll depend on fan demand, of which there's unlikely to be a whole lot. There wasn't that much for her in the first place, hence her role as an Echo Fighter. Ridley's likely never going to go away, but the next Smash game is going to be cutting most of the roster and just going with the bare essentials so that they can have more newcomers. Echo Fighters in general probably won't make the cut.
Dark Samus at least has her foot in the door, though, which is more than most characters can say. Where the future takes us, we don't know. Even if Ultimate is the only game she's in, I'm just satisfied she made it into the best one.
Why does chrom have a different move then for his up b?
Because he could borrow it easily from Ike so it didn't require that much extra work. It wasn't like they gave him a unique move of his own, they just copied it from another character just as they copied everything else from Roy.