I mean they could intentionally be recycling assets for a good chunk of the veterans so they can spend more time making the story mode a big deal. This very well could turn into a Brawl situation where once the dust settles people are left frustrated that so much dev time was wasted on the story mode and it impacted the final base roster.
Again, not really how this works... a story mode like SFV had would add additional cost, but the labor required would not really impact development of the character roster.
The people designing, testing, and balancing playable characters, and making them work within the game's engine, are not really impacted by a story mode. It DOES add more work for the animators, but the increased scope of doing a story mode almost necessitates a larger team working on graphical assets and animations anyway, or even a small secondary team who specifically animate for the story mode cutscenes. So not doing a story mode would likely have just meant fewer animators working on the game.
Other than animation work (addressed previously), there's not a lot of technical work that goes into that kind of story mode, because it's just a series of cutscenes broken up by fights. It isn't necessarily cheap or easy, but that's because it requires significantly more scenario writing, more money and time spent on VAs, etc. And I promise you, the dude working out hitboxes or tweaking damage values on Dante's moveset isn't the same guy scribbling out storyboards for the first scene of Iron Man's chapter.
If anything ends up limiting the size of the roster, it's the relatively short development time. Capcom seems to want to churn this baby out pronto while still being more feature complete than SFV, though they seem far less constrained by their budget (maybe thanks to Marvel's involvement).
