I don't know what character Cervantes really had left to ruin at this point, although making him human did make him completely fucking irrelevant to everything that was going on.
Really, what is Cervantes? As a zombie, he's never even gotten close to reclaiming his old power canonically, Nightmare couldn't give less of a shit what he's up to, and the only character all that concerned with what the guy does is Ivy. He's her grudge-match slot. That's literally his only worthwhile contribution to the series post-death. Frankly, I think that he, Mitsu, and Voldo have got to go from a storyline perspective, because all they can do is go through the motions they've been stuck in for most of the series now and never do anything interesting. Shura's a popular character, and at this point it makes more sense narratively for HER to have Cervantes' movelist than it does for completely-normal-human Cervantes to! Grandfather-clauseing all these characters in really hurts the credibility of the timeskip; I don't think anyone really gave a crap that Voldo was exactly the same 17 years later as he was in 4, or that people wanted to see his infinite loop of trying and failing to steal Soul Edge continue if it meant we never even found out what happened to Talim or Zasalamel or one of those people who didn't make the cut because they weren't in the first game. If they had the balls to kill off Sophitia, some more of the old guys have got to go and be replaced.
I do agree that SCV had too many heroes, though. I don't agree that there's any need to grimdark things up, since Pyrrha's storyline was incredibly dark and Patrokolos isn't exactly mr admirable hero himself, but I do think a stronger villainous presence in the cast is needed, particularly Zasalamel returning. I dunno, maybe give Tira and Nightmare some hangers-on if you need to add some more evil characters.
As for ZWEI, I tolerated him because I assume he's an extension of the Shamanic magic Talim is known to possess in a different form. If we knew literally ANYTHING about him, though, it'd help his cause a lot.