The flaw in this line of thinking I feel, aside from the fact that Ken is technically an echo fighter, is that it erases Chun-Li's value as a standalone character.
In my eyes the initiative for Street Fighter is unique. Unlike how the conversation is posed around Sonic, it's not so much out of a perceived obligation to add another character. Instead it is centralized on Chun-Li, who is herself one of the most important and popular characters absent from the roster period. She is an icon whose crossover appeal is palpable in simply being there to fight Peach or Samus. She is a Capcom mascot equivalent, not secondary to, Mega Man or Ryu.
No disrespect to Zero, but he doesn't really have that. I think he'd be a great choice too, but it's really not as simple as making sure everyone gets their turn first... I mean, if we got a wholly unique Zero, that actually puts Mega Man a cut above Street Fighter for having two unique characters instead of an echo. It's never gonna be a neat and tidy, equally fair distribution of characters so I dunno. I doubt they're too hung up on it. Not saying it can't happen but I think "Zero is more likely because SF has two characters already" is a big oversimplification.